Fair Value Of Financial Instruments (Schedule Of Changes In Fair Value Associated With Level 3 Financial Instruments) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers, Into Level 3		$ 0
Transfers, Out of Level 3		0
Non-Agency [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 1	1
Ending Balance	1	1
Asset-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	357	357
Unrealized gains/(losses), Included in changes in net assets	3	3
Redemptions	(5)	(5)
Net change	(2)	(2)
Ending Balance	355	355
Fixed Income Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	229	242
Redemptions	(229)	(242)
Net change	(229)	(242)
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	320	253
Unrealized gains/(losses), Included in changes in net assets	(197)	(130)
Net change	(197)	(130)
Ending Balance	123	123
Equity Auction Rate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	25,287	34,789
Unrealized gains/(losses), Included in changes in net assets	16	89
Redemptions		(9,575)
Net change	16	(9,486)
Ending Balance	25,303	25,303
Auction Rate Municipal Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	847	846
Unrealized gains/(losses), Included in changes in net assets	(1)
Net change	(1)
Ending Balance	846	846
Other Investment [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	857	857
Ending Balance	$ 857	$ 857