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Marketable Securities (Investments Available For Sale) (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
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Debt Securities, Available-for-sale [Line Items] |  |
Equity securities, FV-NI | $ 5,339 | $ 4,887
Investments Available for sale, Cost Amount | 110,305 | 114,398
Investments Available for sale, Gross Unrealized Gains | 591 | 126
Investments Available for sale, Gross Unrealized (Losses) | (1,072) | (2,623)
Investments Available for sale, Fair Value | 109,824 | 111,901
SEI-sponsored mutual funds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Equity securities, FV-NI, Cost Amount | 7,429 | 7,446
Equity securities, FV-NI, Gross Unrealized Gains | 77 | 0
Equity securities, FV-NI, Gross Unrealized (Losses) | (384) | (788)
Equity securities, FV-NI | 7,122 | 6,658
Equities and other mutual funds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Equity securities, FV-NI, Cost Amount | 3,475 | 3,434
Equity securities, FV-NI, Gross Unrealized Gains | 514 | 126
Equity securities, FV-NI, Gross Unrealized (Losses) | 0 | 0
Equity securities, FV-NI | 3,989 | 3,560
Fixed income securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt securities, Cost Amount | 99,401 | 103,518
Debt securities, Gross Unrealized Gains | 0 | 0
Debt securities, Gross Unrealized (Losses) | (688) | (1,835)
Debt securities, Fair Value | $ 98,713 | $ 101,683

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