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Marketable Securities (Investments Available For Sale) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
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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Debt Securities, Available-For-Sale And Equity Securities, FV-NI

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