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Marketable Securities (Investments Available For Sale) (Details) - USD ($) $ in Thousands | Jun. 30, 2021 | Dec. 31, 2020
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Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale and Equity Securities, Cost | $ 127,647 | $ 102,549
Available For Sale and Equity Securities, Gross Unrealized Gains | 2,392 | 2,870
Available For Sale and Equity Securities, Gross Unrealized (Losses) | 0 | 0
Available For Sale and Equity Securities, Fair Value | 130,039 | 105,419
Available-for-sale debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available for sale debt securities, Cost | 115,818 | 91,262
Available for sale debt securities, Gross Unrealized Gains | 1,490 | 2,015
Available for sale debt securities, Gross Unrealized (Losses) | 0 | 0
Available for sale debt securities, Fair Value | 117,308 | 93,277
SEI-sponsored mutual funds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Equity securities, FV-NI, Cost | 6,896 | 6,866
Equity securities, FV-NI, Gross Unrealized Gains | 666 | 382
Equity securities, FV-NI, Gross Unrealized (Losses) | 0 | 0
Equity securities, FV-NI, Fair Value | 7,562 | 7,248
Equities and other mutual funds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Equity securities, FV-NI, Cost | 4,933 | 4,421
Equity securities, FV-NI, Gross Unrealized Gains | 236 | 473
Equity securities, FV-NI, Gross Unrealized (Losses) | 0 | 0
Equity securities, FV-NI, Fair Value | $ 5,169 | $ 4,894

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