Cash and Securities Segregated Under Federal Regulations and Other Requirements (Details) - USD ($) $ in Billions	Sep. 30, 2021	Dec. 31, 2020
Brokers and Dealers [Abstract]
United States Treasury Bills in special reserve bank custody account for exclusive benefit of brokerage customers	$ 0.9	$ 1.8

Cash and securities deposited in a special reserve account for the exclusive benefit of customers.

No definition available.

Name:	us-gaap_CashAndSecuritiesSegregatedUnderSecuritiesExchangeCommissionRegulation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant