Fair Value Measurements - Classes of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financial Assets
Cash and Cash Equivalents	$ 179,961	$ 156,832
Investments—Consolidated Investment Companies
Equity Securities	8,262	1,902
Debt Securities	13,586	20,896
Investments—Affiliates and Other
Equity Securities	21,244	7,404
Debt Securities	3,957	3,456
Other	765	667
Total Financial Assets	227,775	191,157
Financial Liabilities
Total Financial Liabilities	3,342	4,290
Level 1
Financial Assets
Cash and Cash Equivalents	179,961	156,832
Investments—Consolidated Investment Companies
Equity Securities	4,887	1,269
Debt Securities	0	0
Investments—Affiliates and Other
Equity Securities	21,163	6,963
Debt Securities	0	0
Other	627	597
Total Financial Assets	206,638	165,661
Financial Liabilities
Total Financial Liabilities	22	53
Level 2
Financial Assets
Cash and Cash Equivalents	0	0
Investments—Consolidated Investment Companies
Equity Securities	3,375	633
Debt Securities	13,586	20,896
Investments—Affiliates and Other
Equity Securities	70	403
Debt Securities	3,653	3,456
Other	68	0
Total Financial Assets	20,752	25,388
Financial Liabilities
Total Financial Liabilities	2,301	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	304	0
Other	70	70
Total Financial Assets	385	108
Financial Liabilities
Total Financial Liabilities	$ 1,019	$ 385