Fair Value Measurements - Classes of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financial Assets
Cash and Cash Equivalents	$ 196,513	$ 156,832
Investments—Consolidated Investment Companies
Equity Securities	11,477	1,902
Debt Securities	32,184	20,896
Investments—Affiliates and Other
Equity Securities	65,260	7,404
Debt Securities	3,571	3,456
Other	4,353	667
Total Financial Assets	313,358	191,157
Financial Liabilities
Total Financial Liabilities	4,752	4,290
Level 1
Financial Assets
Cash and Cash Equivalents	196,513	156,832
Investments—Consolidated Investment Companies
Equity Securities	5,473	1,269
Debt Securities	0	0
Investments—Affiliates and Other
Equity Securities	65,183	6,963
Debt Securities	0	0
Other	4,334	597
Total Financial Assets	271,503	165,661
Financial Liabilities
Total Financial Liabilities	31	53
Level 2
Financial Assets
Cash and Cash Equivalents	0	0
Investments—Consolidated Investment Companies
Equity Securities	6,004	633
Debt Securities	32,184	20,896
Investments—Affiliates and Other
Equity Securities	66	403
Debt Securities	3,264	3,456
Other	19	0
Total Financial Assets	41,537	25,388
Financial Liabilities
Total Financial Liabilities	3,192	3,852
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	11	38
Debt Securities	307	0
Other	0	70
Total Financial Assets	318	108
Financial Liabilities
Total Financial Liabilities	$ 1,529	$ 385