Fair Value Of Financial Instruments (Schedule Of Changes In Fair Value Associated With Level 3 Financial Instruments) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020		Dec. 31, 2019
Other financial instrument owned [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 9,326		$ 176
Realized losses	(71)	[1]	451	[2]
Purchases	25,852		24,316
Sales	(20,000)		(16,081)
Redemptions	(697)		(3)
Transfers into Level 3			465
Net change	1,958		9,150
Ending Balance	11,284		9,326
Unrealized gains			2
Corporate Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	1,060
Transfers into Level 3			1,060
Transfers out of Level 3	(1,060)
Net change	(1,060)		1,060
Ending Balance			1,060
Equity Auction Rate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	14,427		17,336
Redemptions			(2,700)
Transfers out of Level 3			(470)
Net change	(1,494)		(2,909)
Ending Balance	12,933		14,427
Unrealized gains			261
Other Investment [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	853		857
Sales	(20)
Redemptions			(4)
Transfers into Level 3	41,576
Net change	41,260		(4)
Ending Balance	$ 42,113		$ 853

[1] Realized and unrealized losses related to financial instruments owned and investments are reported in other income in the consolidated statements of operations

[2] Realized and unrealized gains/(losses) related to financial instruments owned and investments are reported in other income in the consolidated statements of operations