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Marketable Securities (Investments Available For Sale) (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
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Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale and Equity Securities, Cost | $ 138,588 | $ 102,549
Available For Sale and Equity Securities, Gross Unrealized Gains | 1,491 | 2,870
Available For Sale and Equity Securities, Gross Unrealized (Losses) | 0 | 0
Available For Sale and Equity Securities, Fair Value | 140,079 | 105,419
Available-for-sale debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available for sale debt securities, Cost | 126,964 | 91,262
Available for sale debt securities, Gross Unrealized Gains | 874 | 2,015
Available for sale debt securities, Gross Unrealized (Losses) | 0 | 0
Available for sale debt securities, Fair Value | 127,838 | 93,277
SEI-sponsored mutual funds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Equity securities, FV-NI, Cost | 6,692 | 6,866
Equity securities, FV-NI, Gross Unrealized Gains | 380 | 382
Equity securities, FV-NI, Gross Unrealized (Losses) | 0 | 0
Equity securities, FV-NI, Fair Value | 7,072 | 7,248
Equities and other mutual funds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Equity securities, FV-NI, Cost | 4,932 | 4,421
Equity securities, FV-NI, Gross Unrealized Gains | 237 | 473
Equity securities, FV-NI, Gross Unrealized (Losses) | 0 | 0
Equity securities, FV-NI, Fair Value | $ 5,169 | $ 4,894

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