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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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