Fair Value Measurements - Changes in Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$ 72,993	$ 48,620
Other Investments [Abstract]
Total assets measured at fair value	169,462	129,335
Sponsored funds
Marketable securities trading:
Marketable securities trading	46,849	60,019
Equity securities
Marketable securities trading:
Marketable securities trading	15,810	11,473
Sponsored closed-end funds
Marketable securities available-for-sale:
Marketable securities available for sale	3,765	3,415
Collateralized loan obligations
Other Investments [Abstract]
Investments in collateralized loan obligations	23,339
Nonqualified retirement plan assets
Other Investments [Abstract]
Other investments	6,706	5,808
Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	72,993	48,620
Other Investments [Abstract]
Total assets measured at fair value	146,123	129,335
Level 1 | Sponsored funds
Marketable securities trading:
Marketable securities trading	46,849	60,019
Level 1 | Equity securities
Marketable securities trading:
Marketable securities trading	15,810	11,473
Level 1 | Sponsored closed-end funds
Marketable securities available-for-sale:
Marketable securities available for sale	3,765	3,415
Level 1 | Collateralized loan obligations
Other Investments [Abstract]
Investments in collateralized loan obligations	0
Level 1 | Nonqualified retirement plan assets
Other Investments [Abstract]
Other investments	6,706	5,808
Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	0	0
Other Investments [Abstract]
Total assets measured at fair value	18,900	0
Level 2 | Sponsored funds
Marketable securities trading:
Marketable securities trading	0	0
Level 2 | Equity securities
Marketable securities trading:
Marketable securities trading	0	0
Level 2 | Sponsored closed-end funds
Marketable securities available-for-sale:
Marketable securities available for sale	0	0
Level 2 | Collateralized loan obligations
Other Investments [Abstract]
Investments in collateralized loan obligations	18,900
Level 2 | Nonqualified retirement plan assets
Other Investments [Abstract]
Other investments	0	0
Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	0	0
Other Investments [Abstract]
Total assets measured at fair value	4,439	0
Level 3 | Sponsored funds
Marketable securities trading:
Marketable securities trading	0	0
Level 3 | Equity securities
Marketable securities trading:
Marketable securities trading	0	0
Level 3 | Sponsored closed-end funds
Marketable securities available-for-sale:
Marketable securities available for sale	0	0
Level 3 | Collateralized loan obligations
Other Investments [Abstract]
Investments in collateralized loan obligations	4,439
Level 3 | Nonqualified retirement plan assets
Other Investments [Abstract]
Other investments	$ 0	$ 0