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, 2018 USD ($)
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
Ending Balance	1
Asset-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	357
Ending Balance	357
Fixed Income Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	242
Redemptions	(13)
Net change	(13)
Ending Balance	229
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	253
Unrealized gains/(losses), Included in changes in net assets	67
Net change	67
Ending Balance	320
Equity Auction Rate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	34,789
Unrealized gains/(losses), Included in changes in net assets	73
Redemptions	(9,575)
Net change	(9,502)
Ending Balance	25,287
Auction Rate Municipal Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	846
Unrealized gains/(losses), Included in changes in net assets	1
Net change	1
Ending Balance	847
Other Investment [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	857
Ending Balance	$ 857