Fair Value Measurements - Classes of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financial Assets
Cash and Cash Equivalents	$ 156,832	$ 316,264
Investments—Consolidated Investment Companies
Equity Securities	1,902	6,170
Debt Securities	20,896	39,241
Investments—Affiliates and Other
Equity Securities	7,404	4,866
Debt Securities	3,456	2,997
Other	667	1,240
Total Financial Assets	191,157	370,778
Total Financial Liabilities	4,290	1,203
Level 1
Financial Assets
Cash and Cash Equivalents	156,832	205,364
Investments—Consolidated Investment Companies
Equity Securities	1,269	5,424
Debt Securities	0	0
Investments—Affiliates and Other
Equity Securities	6,684	4,564
Debt Securities	0	0
Other	597	123
Total Financial Assets	165,382	215,475
Total Financial Liabilities	53	0
Level 2
Financial Assets
Cash and Cash Equivalents	0	0
Investments—Consolidated Investment Companies
Equity Securities	633	746
Debt Securities	20,896	39,241
Investments—Affiliates and Other
Equity Securities	403	0
Debt Securities	3,456	2,997
Other	0	357
Total Financial Assets	25,388	43,341
Total Financial Liabilities	3,852	0
Level 3
Financial Assets
Cash and Cash Equivalents	0	0
Investments—Consolidated Investment Companies
Equity Securities	0	0
Debt Securities	0	0
Investments—Affiliates and Other
Equity Securities	38	0
Debt Securities	0	0
Other	70	760
Total Financial Assets	108	760
Total Financial Liabilities	385	1,203
NAV Practical Expedient
Financial Assets
Cash and Cash Equivalents		110,900
Investments—Affiliates and Other
Equity Securities	279	302
Total Financial Assets	$ 279	$ 111,202