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Fair Value Of Financial Instruments (Schedule Of Changes In Fair Value Associated With Level 3 Financial Instruments) (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
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 |
Ending Balance | 1 | $ 1
Asset-Backed Securities [Member] |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Beginning Balance | 357 |
Unrealized gains/(losses), Included in changes in net assets | (164) |
Redemptions | (18) |
Net change | (182) |
Ending Balance | 175 | 357
Equity Auction Rate Securities [Member] |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Beginning Balance | 34,789 | 48,689
Unrealized gains/(losses), Included in changes in net assets | 1,193 | 785
Redemptions | (19,350) |
Transfers, Out of Level 3 |  | (14,685)
Net change | (18,157) | (13,900)
Ending Balance | 16,632 | 34,789
Corporate Fixed Income Securities [Member] |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Beginning Balance | 242 | 273
Redemptions | (242) | (31)
Net change | (242) | (31)
Ending Balance |  | 242
Auction Rate Municipal Securities [Member] |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Beginning Balance | 846 | 832
Unrealized gains/(losses), Included in changes in net assets | 8 | 14
Redemptions | (150) |
Net change | (142) | 14
Ending Balance | 704 | 846
Other Investment [Member] |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Beginning Balance | 857 | 1,240
Realized gains/(losses) |  | (383)
Net change |  | (383)
Ending Balance | 857 | 857
Non-Agency and Asset-backed Securities [Member] |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Beginning Balance | 358 | 1,082
Unrealized gains/(losses), Included in changes in net assets |  | (260)
Realized gains/(losses) |  | 90
Sales |  | (324)
Redemptions |  | (230)
Net change |  | (724)
Ending Balance |  | 358
Corporate Equity Securities [Member] |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Beginning Balance |  | 3,833
Unrealized gains/(losses), Included in changes in net assets |  | (133)
Realized gains/(losses) |  | 13
Sales |  | (120)
Transfers, Out of Level 3 |  | (3,593)
Net change |  | (3,833)
Equity Securities [Member] |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Beginning Balance | 253 | 600
Unrealized gains/(losses), Included in changes in net assets | (130) | (88)
Realized gains/(losses) | 21 | (259)
Sales | (144) |
Net change | $ (253) | (347)
Ending Balance |  | $ 253

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