Fair Value Of Financial Instruments (Schedule Of Changes In Fair Value Associated With Level 3 Financial Instruments) (Details)	3 Months Ended
	Mar. 31, 2017 USD ($)
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	1,082,000
Realized gains	1,000
Redemptions	(64,000)
Net change	(63,000)
Ending Balance	1,019,000
Corporate Fixed Income Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	273,000
Redemptions	(6,000)
Net change	(6,000)
Ending Balance	267,000
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	600,000
Unrealized gains/(losses), Included in changes in net assets	(17,000)
Net change	(17,000)
Ending Balance	583,000
Corporate Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	3,833,000
Ending Balance	3,833,000
Equity Auction Rate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	48,689,000
Unrealized gains/(losses), Included in changes in net assets	366,000
Net change	366,000
Ending Balance	49,055,000
Auction Rate Municipal Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	832,000
Unrealized gains/(losses), Included in changes in net assets	5,000
Net change	5,000
Ending Balance	837,000
Other Investment [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	1,240,000
Ending Balance	$ 1,240,000