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Marketable Securities (Investments Available For Sale) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Available For Sale and Equity Securities, Cost	$ 115,394	$ 114,398
Available For Sale and Equity Securities, Gross Unrealized Gains	1,562	126
Available For Sale and Equity Securities, Gross Unrealized (Losses)	(39)	(2,623)
Available For Sale and Equity Securities, Fair Value	116,917	111,901
Available for sale debt securities
Debt Securities, Available-for-sale [Line Items]
Available for sale debt securities, Cost	104,193	103,518
Available for sale debt securities, Gross Unrealized Gains	605	0
Available for sale debt securities, Gross Unrealized (Losses)	0	(1,835)
Available for sale debt securities, Fair Value	104,798	101,683
SEI-sponsored mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, Cost	7,564	7,446
Equity securities, FV-NI, Gross Unrealized Gains	125	0
Equity securities, FV-NI, Gross Unrealized (Losses)	(39)	(788)
Equity securities, FV-NI, Fair Value	7,650	6,658
Equities and other mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, FV-NI, Cost	3,637	3,434
Equity securities, FV-NI, Gross Unrealized Gains	832	126
Equity securities, FV-NI, Gross Unrealized (Losses)	0	0
Equity securities, FV-NI, Fair Value	$ 4,469	$ 3,560

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