Revenue Recognition (Schedule of Performance Obligation) (Details)	9 Months Ended
Sep. 30, 2020
One [Member]
Revenue Stream	Commissions and fees, Principal transactions, Market making, Stock borrow / stock loan, Advisory fees
Performance Obligation	Provide financial services to customers and counterparties
Two [Member]
Revenue Stream	Margin interest, marketing and distribution fees, Interest income, Other income
Performance Obligation	n/a

Description of good or service to be transferred in contract with customer. Includes, but is not limited to, arrangement for another party to transfer good or service to customer.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 12
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130554-203045

Name:	us-gaap_RevenuePerformanceObligationDescriptionOfGoodOrService
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration