Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets
Cash equivalents	$ 138,301	$ 72,993
Marketable securities:
Marketable securities trading	62,330	66,424
Other investments:
Total assets measured at fair value	213,083	169,462
Sponsored funds
Marketable securities:
Marketable securities trading	39,951	46,849
Equity securities
Marketable securities:
Marketable securities trading	18,641	15,810
Sponsored closed-end funds
Marketable securities:
Marketable securities trading	3,738	3,765
Collateralized loan obligation
Other investments:
Investments in collateralized loan obligations	5,043	23,339
Nonqualified retirement plan assets
Other investments:
Nonqualified retirement plan assets	7,409	6,706
Level 1
Assets
Cash equivalents	138,301	72,993
Other investments:
Total assets measured at fair value	208,040	146,123
Level 1 | Sponsored funds
Marketable securities:
Marketable securities trading	39,951	46,849
Level 1 | Equity securities
Marketable securities:
Marketable securities trading	18,641	15,810
Level 1 | Sponsored closed-end funds
Marketable securities:
Marketable securities trading	3,738	3,765
Level 1 | Collateralized loan obligation
Other investments:
Investments in collateralized loan obligations	0	0
Level 1 | Nonqualified retirement plan assets
Other investments:
Nonqualified retirement plan assets	7,409	6,706
Level 2
Assets
Cash equivalents	0	0
Other investments:
Total assets measured at fair value	0	18,900
Level 2 | Sponsored funds
Marketable securities:
Marketable securities trading	0	0
Level 2 | Equity securities
Marketable securities:
Marketable securities trading	0	0
Level 2 | Sponsored closed-end funds
Marketable securities:
Marketable securities trading	0	0
Level 2 | Collateralized loan obligation
Other investments:
Investments in collateralized loan obligations	0	18,900
Level 2 | Nonqualified retirement plan assets
Other investments:
Nonqualified retirement plan assets	0	0
Level 3
Assets
Cash equivalents	0	0
Other investments:
Total assets measured at fair value	5,043	4,439
Level 3 | Sponsored funds
Marketable securities:
Marketable securities trading	0	0
Level 3 | Equity securities
Marketable securities:
Marketable securities trading	0	0
Level 3 | Sponsored closed-end funds
Marketable securities:
Marketable securities trading	0	0
Level 3 | Collateralized loan obligation
Other investments:
Investments in collateralized loan obligations	5,043	4,439
Level 3 | Nonqualified retirement plan assets
Other investments:
Nonqualified retirement plan assets	$ 0	$ 0