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Marketable Securities (Investments Available For Sale) (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Equity Securities: |  |
Fair Value | $ 11,042 | $ 11,250
Debt Securities: |  |
Fair Value | 77,977 | 76,733
Cost Amount | 91,258 | 88,570
Gross Unrealized Gains | 548 | 568
Gross Unrealized (Losses) | 2,787 | 1,155
Fair Value | 89,019 | 87,983
SEI-sponsored mutual funds |  |
Equity Securities: |  |
Cost Amount | 7,388 | 7,369
Gross Unrealized Gains | 96 | 110
Gross Unrealized (Losses) | 355 | 143
Fair Value | 7,129 | 7,336
Equities and other mutual funds |  |
Equity Securities: |  |
Cost Amount | 3,461 | 3,456
Gross Unrealized Gains | 452 | 458
Gross Unrealized (Losses) | 0 | 0
Fair Value | 3,913 | 3,914
Debt securities |  |
Debt Securities: |  |
Cost Amount | 80,409 | 77,745
Gross Unrealized Gains | 0 | 0
Gross Unrealized (Losses) | (2,432) | (1,012)
Fair Value | $ 77,977 | $ 76,733

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