Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets
Cash equivalents	$ 43,715	$ 72,993
Marketable securities:
Marketable securities trading	62,039	66,424
Other investments:
Total assets measured at fair value	155,718	169,462
Sponsored funds
Marketable securities:
Marketable securities trading	41,915	46,849
Equity securities
Marketable securities:
Marketable securities trading	16,549	15,810
Sponsored closed-end funds
Marketable securities:
Marketable securities trading	3,575	3,765
Collateralized loan obligation
Other investments:
Investment in collateralized loan obligations	43,295	23,339
Nonqualified retirement plan assets
Other investments:
Nonqualified retirement plan assets	6,669	6,706
Level 1
Assets
Cash equivalents	43,715	72,993
Other investments:
Total assets measured at fair value	112,423	146,123
Level 1 | Sponsored funds
Marketable securities:
Marketable securities trading	41,915	46,849
Level 1 | Equity securities
Marketable securities:
Marketable securities trading	16,549	15,810
Level 1 | Sponsored closed-end funds
Marketable securities:
Marketable securities trading	3,575	3,765
Level 1 | Collateralized loan obligation
Other investments:
Investment in collateralized loan obligations	0	0
Level 1 | Nonqualified retirement plan assets
Other investments:
Nonqualified retirement plan assets	6,669	6,706
Level 2
Assets
Cash equivalents	0	0
Other investments:
Total assets measured at fair value	37,763	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:
Investment in collateralized loan obligations	37,763	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	5,532	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:
Investment in collateralized loan obligations	5,532	4,439
Level 3 | Nonqualified retirement plan assets
Other investments:
Nonqualified retirement plan assets	$ 0	$ 0