Fair Value Of Financial Instruments (Schedule Of Changes In Fair Value Associated With Level 3 Financial Instruments) (Details) - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers, Into Level 3		$ 0
Transfers, Out of Level 3	$ (18,300,000)	(18,300,000)
Non-Agency [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	1,019,000	1,082,000
Unrealized gains/(losses), Included in changes in net assets	(100,000)	(100,000)
Realized gains/(losses)	96,000	97,000
Sales	(324,000)	(324,000)
Redemptions	(166,000)	(230,000)
Net change	(494,000)	(557,000)
Ending Balance	525,000	525,000
Fixed Income Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	267,000	273,000
Redemptions	(11,000)	(17,000)
Net change	(11,000)	(17,000)
Ending Balance	256,000	256,000
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	583,000	600,000
Unrealized gains/(losses), Included in changes in net assets	(216,000)	(233,000)
Net change	(216,000)	(233,000)
Ending Balance	367,000	367,000
Corporate Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	3,833,000	3,833,000
Transfers, Out of Level 3	(3,593,000)	(3,593,000)
Net change	(3,593,000)	(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	49,055,000	48,689,000
Unrealized gains/(losses), Included in changes in net assets	251,000	617,000
Transfers, Out of Level 3	(14,685,000)	(14,685,000)
Net change	(14,434,000)	(14,068,000)
Ending Balance	34,621,000	34,621,000
Auction Rate Municipal Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	837,000	832,000
Unrealized gains/(losses), Included in changes in net assets	4,000	9,000
Net change	4,000	9,000
Ending Balance	841,000	841,000
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