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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, 2020 | Dec. 31, 2019
Other financial instrument owned [Member] |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Beginning Balance | $ 9,326 |  | $ 176 |
Realized losses | (71) | [1] | 451 | [2]
Purchases | 25,852 |  | 24,316 |
Sales | (20,000) |  | (16,081) |
Redemptions | (697) |  | (3) |
Transfers into Level 3 |  |  | 465 |
Net change | 1,958 |  | 9,150 |
Ending Balance | 11,284 |  | 9,326 |
Unrealized gains |  |  | 2 |
Corporate Equity Securities [Member] |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Beginning Balance | 1,060 |  |  |
Transfers into Level 3 |  |  | 1,060 |
Transfers out of Level 3 | (1,060) |  |  |
Net change | (1,060) |  | 1,060 |
Ending Balance |  |  | 1,060 |
Equity Auction Rate Securities [Member] |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Beginning Balance | 14,427 |  | 17,336 |
Redemptions |  |  | (2,700) |
Transfers out of Level 3 |  |  | (470) |
Net change | (1,494) |  | (2,909) |
Ending Balance | 12,933 |  | 14,427 |
Unrealized gains |  |  | 261 |
Other Investment [Member] |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Beginning Balance | 853 |  | 857 |
Sales | (20) |  |  |
Redemptions |  |  | (4) |
Transfers into Level 3 | 41,576 |  |  |
Net change | 41,260 |  | (4) |
Ending Balance | $ 42,113 |  | $ 853 |
[1] | Realized and unrealized losses related to financial instruments owned and investments are reported in other income in the consolidated statements of operations
[2] | Realized and unrealized gains/(losses) related to financial instruments owned and investments are reported in other income in the consolidated statements of operations

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- Definition Fair value measurement with unobservable inputs reconciliation recurring basis realized asset gain loss included in earnings. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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