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, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers, Into Level 3	$ 0
Non-Agency [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	1
Ending Balance	1	$ 1
Asset-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	357
Unrealized gains/(losses), Included in changes in net assets	(164)
Redemptions	(18)
Net change	(182)
Ending Balance	175	357
Equity Auction Rate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	34,789	48,689
Unrealized gains/(losses), Included in changes in net assets	1,193	785
Redemptions	(19,350)
Transfers, Out of Level 3		(14,685)
Net change	(18,157)	(13,900)
Ending Balance	16,632	34,789
Corporate Fixed Income Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	242	273
Redemptions	(242)	(31)
Net change	(242)	(31)
Ending Balance		242
Auction Rate Municipal Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	846	832
Unrealized gains/(losses), Included in changes in net assets	8	14
Redemptions	(150)
Net change	(142)	14
Ending Balance	704	846
Other Investment [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	857	1,240
Realized gains/(losses)		(383)
Net change		(383)
Ending Balance	857	857
Non-Agency and Asset-backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	358	1,082
Unrealized gains/(losses), Included in changes in net assets		(260)
Realized gains/(losses)		90
Sales		(324)
Redemptions		(230)
Net change		(724)
Ending Balance		358
Corporate Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance		3,833
Unrealized gains/(losses), Included in changes in net assets		(133)
Realized gains/(losses)		13
Sales		(120)
Transfers, Out of Level 3		(3,593)
Net change		(3,833)
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	253	600
Unrealized gains/(losses), Included in changes in net assets	(130)	(88)
Realized gains/(losses)	21	(259)
Sales	(144)
Net change	$ (253)	(347)
Ending Balance		$ 253