| r0.1 | label_boi_a2/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | N/A | boi_CS:x2 | N/A | boi_MC:x125 | N/A | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| r0.2 | label_boi_a2/Company | boi_met:md13 | xbrli:monetaryItemType | instant | N/A | boi_CS:x1 | N/A | boi_MC:x125 | N/A | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| r1.1 | Interest income/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x2 | N/A | boi_MC:x125 | N/A | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| r1.2 | Interest income/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x1 | N/A | boi_MC:x125 | N/A | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 1.1 | From credit to cardholders/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x2 | N/A | boi_MC:x125 | boi_MC:x8 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | boi_SE:x122 |
| 1.2 | From credit to cardholders/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x1 | N/A | boi_MC:x125 | boi_MC:x8 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | boi_SE:x122 |
| 2.1 | From credit to businesses/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x2 | N/A | boi_MC:x125 | boi_MC:x8 | boi_MU:x1 | N/A | boi_RP:x25 | boi_SE:x88 | boi_SE:x123 |
| 2.2 | From credit to businesses/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x1 | N/A | boi_MC:x125 | boi_MC:x8 | boi_MU:x1 | N/A | boi_RP:x25 | boi_SE:x88 | boi_SE:x123 |
| 3.1 | From credit to others/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x2 | N/A | boi_MC:x125 | boi_MC:x8 | boi_MU:x1 | N/A | boi_RP:x25 | boi_SE:x167 | boi_SE:x123 |
| 3.2 | From credit to others/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x1 | N/A | boi_MC:x125 | boi_MC:x8 | boi_MU:x1 | N/A | boi_RP:x25 | boi_SE:x167 | boi_SE:x123 |
| 4.1 | From bank deposits/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x2 | N/A | boi_MC:x125 | boi_MC:x9 | boi_MU:x1 | N/A | boi_RP:x25 | boi_SE:x1 | boi_SE:x123 |
| 4.2 | From bank deposits/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x1 | N/A | boi_MC:x125 | boi_MC:x9 | boi_MU:x1 | N/A | boi_RP:x25 | boi_SE:x1 | boi_SE:x123 |
| 5.1 | From securities borrowed or acquired under repurchase agreements/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x2 | N/A | boi_MC:x125 | boi_MC:x28 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 5.2 | From securities borrowed or acquired under repurchase agreements/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x1 | N/A | boi_MC:x125 | boi_MC:x28 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 6.1 | From bonds/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x2 | N/A | boi_MC:x125 | boi_MC:x15 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 6.2 | From bonds/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x1 | N/A | boi_MC:x125 | boi_MC:x15 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 7.1 | From other assets/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x2 | N/A | boi_MC:x125 | boi_MC:x443 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 7.2 | From other assets/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x1 | N/A | boi_MC:x125 | boi_MC:x443 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 8.1 | Total interest income/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x2 | N/A | boi_MC:x125 | N/A | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 8.2 | Total interest income/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x1 | N/A | boi_MC:x125 | N/A | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| r10.1 | Interest expenses/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x10 | boi_CS:x2 | N/A | boi_MC:x125 | N/A | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| r10.2 | Interest expenses/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x10 | boi_CS:x1 | N/A | boi_MC:x125 | N/A | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 9.1 | To banking corrporations/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x10 | boi_CS:x2 | N/A | boi_MC:x125 | boi_MC:x17 | boi_MU:x1 | N/A | boi_RP:x25 | boi_SE:x1 | N/A |
| 9.2 | To banking corrporations/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x10 | boi_CS:x1 | N/A | boi_MC:x125 | boi_MC:x17 | boi_MU:x1 | N/A | boi_RP:x25 | boi_SE:x1 | N/A |
| 10.1 | From securities lent or sold under repurchase agreements/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x10 | boi_CS:x2 | N/A | boi_MC:x125 | boi_MC:x28 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 10.2 | From securities lent or sold under repurchase agreements/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x10 | boi_CS:x1 | N/A | boi_MC:x125 | boi_MC:x28 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 11.1 | On bonds/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x10 | boi_CS:x2 | N/A | boi_MC:x125 | boi_MC:x15 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 11.2 | On bonds/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x10 | boi_CS:x1 | N/A | boi_MC:x125 | boi_MC:x15 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 12.1 | On other liabilities/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x10 | boi_CS:x2 | N/A | boi_MC:x125 | boi_MC:x924 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 12.2 | On other liabilities/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x10 | boi_CS:x1 | N/A | boi_MC:x125 | boi_MC:x924 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 13.1 | Total interest expenses/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x10 | boi_CS:x2 | N/A | boi_MC:x125 | N/A | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 13.2 | Total interest expenses/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x10 | boi_CS:x1 | N/A | boi_MC:x125 | N/A | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 14.1 | Total interest income (expenses), net/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x11 | boi_CS:x2 | N/A | boi_MC:x125 | N/A | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 14.2 | Total interest income (expenses), net/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x11 | boi_CS:x1 | N/A | boi_MC:x125 | N/A | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| r17.1 | Breakdown of net effect of derivatives/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | N/A | boi_CS:x2 | boi_EF:x2 | boi_MC:x125 | N/A | boi_MU:x1 | boi_PL:x11 | boi_RP:x25 | N/A | N/A |
| r17.2 | Breakdown of net effect of derivatives/Company | boi_met:md13 | xbrli:monetaryItemType | instant | N/A | boi_CS:x1 | boi_EF:x2 | boi_MC:x125 | N/A | boi_MU:x1 | boi_PL:x11 | boi_RP:x25 | N/A | N/A |
| 15.1 | Interest income:/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x2 | boi_EF:x2 | boi_MC:x125 | N/A | boi_MU:x1 | boi_PL:x11 | boi_RP:x25 | N/A | N/A |
| 15.2 | Interest income:/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x1 | boi_EF:x2 | boi_MC:x125 | N/A | boi_MU:x1 | boi_PL:x11 | boi_RP:x25 | N/A | N/A |
| 16.1 | Interest expenses:/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x10 | boi_CS:x2 | boi_EF:x2 | boi_MC:x125 | N/A | boi_MU:x1 | boi_PL:x11 | boi_RP:x25 | N/A | N/A |
| 16.2 | Interest expenses:/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x10 | boi_CS:x1 | boi_EF:x2 | boi_MC:x125 | N/A | boi_MU:x1 | boi_PL:x11 | boi_RP:x25 | N/A | N/A |
| r20.1 | Breakdown of interest income on bonds, accrual basis/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x2 | N/A | boi_MC:x125 | boi_MC:x15 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| r20.2 | Breakdown of interest income on bonds, accrual basis/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x1 | N/A | boi_MC:x125 | boi_MC:x15 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 17.1 | Held to maturity/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x2 | N/A | boi_MC:x125 | boi_MC:x15 | boi_MU:x1 | boi_PL:x5 | boi_RP:x25 | N/A | N/A |
| 17.2 | Held to maturity/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x1 | N/A | boi_MC:x125 | boi_MC:x15 | boi_MU:x1 | boi_PL:x5 | boi_RP:x25 | N/A | N/A |
| 18.1 | Available for sale/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x2 | N/A | boi_MC:x125 | boi_MC:x15 | boi_MU:x1 | boi_PL:x1 | boi_RP:x25 | N/A | N/A |
| 18.2 | Available for sale/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x1 | N/A | boi_MC:x125 | boi_MC:x15 | boi_MU:x1 | boi_PL:x1 | boi_RP:x25 | N/A | N/A |
| 19.1 | For trading/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x2 | N/A | boi_MC:x125 | boi_MC:x15 | boi_MU:x1 | boi_PL:x2 | boi_RP:x25 | N/A | N/A |
| 19.2 | For trading/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x1 | N/A | boi_MC:x125 | boi_MC:x15 | boi_MU:x1 | boi_PL:x2 | boi_RP:x25 | N/A | N/A |
| 20.1 | Total included in interest income/Consolidated | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x2 | N/A | boi_MC:x125 | boi_MC:x15 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |
| 20.2 | Total included in interest income/Company | boi_met:md13 | xbrli:monetaryItemType | instant | boi_BC:x9 | boi_CS:x1 | N/A | boi_MC:x125 | boi_MC:x15 | boi_MU:x1 | N/A | boi_RP:x25 | N/A | N/A |