| Amount of unrealized loss on investments in held-to-maturity securities. | en-US | http://www.xbrl.org/2003/role/documentation | http://www.xbrl.org/2003/role/link |
| Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss (Deprecated 2018-01-31) | en-US | http://www.xbrl.org/2003/role/label | http://www.xbrl.org/2003/role/link |
| Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss, Total (Deprecated 2018-01-31) | en-US | http://www.xbrl.org/2003/role/totalLabel | http://www.xbrl.org/2003/role/link |