Cash and Securities Segregated Under Federal Regulations and Other Requirements (Details) - USD ($) $ in Billions	Mar. 31, 2018	Dec. 31, 2017
Brokers and Dealers [Abstract]
United States Treasury Bills in special reserve bank custody account for exclusive benefit of brokerage customers of SCB LLC	$ 1.0	$ 0.8

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 305
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6478312&loc=d3e61473-112792

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