Revenue Recognition (Schedule of Loss Related to Principal Transactions) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Principal transactions and proprietary trading
Realized and unrealized gain on primarily riskless principal transactions	$ 1,919,000	$ 4,254,000
Realized and unrealized gain on primarily riskless principal transactions, year over year decrease	(2,335,000)
Unrealized loss on U.S. government securities	(2,186,000)	(6,000)
Unrealized loss on portfolio of U.S. government securities, year over year decrease	(2,180,000)
Total Principal transactions and proprietary trading	(267,000)	$ 4,248,000
Total Principal transactions and proprietary trading, year over year decrease	$ (4,515,000)