Fair Value Measurements - Classes of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financial Assets
Investments—Consolidated Investment Companies	$ 71,103	$ 64,526
Investments—Affiliates and Other	43,209	26,935
Fair Value, Measurements, Recurring
Financial Assets
Cash and Cash Equivalents	322,847	249,174
Investments—Consolidated Investment Companies	71,103	64,526
Investments—Affiliates and Other	43,209	26,935
Other	9,953	6,078
Total Financial Assets	447,112	346,713
Financial Liabilities
Total Financial Liabilities	11,689	2,087
Fair Value, Measurements, Recurring | Level 1
Financial Assets
Cash and Cash Equivalents	322,847	249,174
Investments—Consolidated Investment Companies	13,098	7,245
Investments—Affiliates and Other	37,357	23,667
Other	8,796	2,901
Total Financial Assets	382,098	282,987
Financial Liabilities
Total Financial Liabilities	37	6
Fair Value, Measurements, Recurring | Level 2
Financial Assets
Cash and Cash Equivalents	0	0
Investments—Consolidated Investment Companies	58,005	57,281
Investments—Affiliates and Other	5,497	2,945
Other	1,157	3,177
Total Financial Assets	64,659	63,403
Financial Liabilities
Total Financial Liabilities	6	0
Fair Value, Measurements, Recurring | Level 3
Financial Assets
Cash and Cash Equivalents	0	0
Investments—Consolidated Investment Companies	0	0
Investments—Affiliates and Other	355	323
Other	0	0
Total Financial Assets	355	323
Financial Liabilities
Total Financial Liabilities	$ 11,646	$ 2,081