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Marketable Securities (Investments Available For Sale) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Equity Securities:
Fair Value	$ 11,326	$ 11,250
Debt Securities:
Fair Value	72,972	76,733
Cost Amount	86,544	88,570
Gross Unrealized Gains	759	568
Gross Unrealized (Losses)	(3,005)	(1,155)
Fair Value	84,298	87,983
SEI-sponsored mutual funds
Equity Securities:
Cost Amount	7,392	7,369
Gross Unrealized Gains	107	110
Gross Unrealized (Losses)	(304)	(143)
Fair Value	7,195	7,336
Equities and other mutual funds
Equity Securities:
Cost Amount	3,479	3,456
Gross Unrealized Gains	652	458
Gross Unrealized (Losses)	0	0
Fair Value	4,131	3,914
Debt securities
Debt Securities:
Cost Amount	75,673	77,745
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(2,701)	(1,012)
Fair Value	$ 72,972	$ 76,733

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