IndexAnnual Membership Fees [Member]
| Name | AnnualMembershipFeesMember |
|---|---|
| Namespace | http://fasb.org/us-gaap/2020-01-31 |
| Prefix | us-gaap |
| Data type | nonnum:domainItemType |
| Period type | duration |
| Substitution Group | xbrli:item |
| Balance | None |
| Nillable | True |
| Abstract | True |
Labels
| Text | Lang | Role | Container role |
|---|---|---|---|
| Annual Membership Fees [Member] | en-US | http://www.xbrl.org/2003/role/label | http://www.xbrl.org/2003/role/link |
| Annual membership fees collected in advance and recognized in revenue, net of estimated refunds. | en-US | http://www.xbrl.org/2003/role/documentation | http://www.xbrl.org/2003/role/link |