Derivative Financial Instruments (Statement Of Operations Information) (Details) - Not Designated as Hedging Instruments [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Derivative [Line Items]
Derivative financial instruments, net gain (loss) recognized	$ (212)	$ 777
Revenues - principal transactions and other income [Member] | Foreign currency forward contracts [Member]
Derivative [Line Items]
Derivative financial instruments, net gain (loss) recognized		40
Revenues - net trading [Member] | Other Extended Settlement Trades [Member]
Derivative [Line Items]
Derivative financial instruments, net gain (loss) recognized	1	(24)
Revenues - net trading [Member] | TBAs [Member]
Derivative [Line Items]
Derivative financial instruments, net gain (loss) recognized	$ (213)	$ 761