Fair Value Of Financial Instruments (Schedule Of Changes In Fair Value Associated With Level 3 Financial Instruments) (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers, Into Level 3		$ 0
Transfers, Out of Level 3	$ 0	(18,300,000)
Non-Agency [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	525,000	1,082,000
Unrealized gains/(losses), Included in changes in net assets	(160,000)	(260,000)
Realized gains/(losses)	(7,000)	90,000
Sales		(324,000)
Redemptions		(230,000)
Net change	(167,000)	(724,000)
Ending Balance	358,000	358,000
Fixed Income Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	256,000	273,000
Redemptions	(8,000)	(25,000)
Net change	(8,000)	(25,000)
Ending Balance	248,000	248,000
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	367,000	600,000
Unrealized gains/(losses), Included in changes in net assets	159,000	(74,000)
Realized gains/(losses)	(259,000)	(259,000)
Net change	(100,000)	(333,000)
Ending Balance	267,000	267,000
Corporate Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	240,000	3,833,000
Transfers, Out of Level 3		(3,593,000)
Net change		(3,593,000)
Ending Balance	240,000	240,000
Equity Auction Rate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	34,621,000	48,689,000
Unrealized gains/(losses), Included in changes in net assets	(13,000)	604,000
Transfers, Out of Level 3		(14,685,000)
Net change	(13,000)	(14,081,000)
Ending Balance	34,608,000	34,608,000
Auction Rate Municipal Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	841,000	832,000
Unrealized gains/(losses), Included in changes in net assets	(1,000)	8,000
Net change	(1,000)	8,000
Ending Balance	840,000	840,000
Other Investment [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	1,240,000	1,240,000
Ending Balance	$ 1,240,000	$ 1,240,000