Fair Value Of Financial Instruments (Schedule Of Changes In Fair Value Associated With Level 3 Financial Instruments) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($)
Other financial instrument owned [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 11,284
Unrealized gains/(losses)	(21)
Purchases	3,014
Net change	2,993
Ending Balance	14,277
Corporate Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Purchases	2,000
Net change	2,000
Ending Balance	2,000
Equity Auction Rate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	12,933
Unrealized gains/(losses)	26
Net change	26
Ending Balance	12,959
Other Investment [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	42,113
Unrealized gains/(losses)	(285)
Net change	(285)
Ending Balance	$ 41,828