Goodwill and Other Intangible Assets - Gross Cost and Accumulated Amortization of Other Intangible Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, Gross Cost	$ 69,777	$ 69,171
Other intangible assets, accumulated amortization	64,681	63,099
Other intangible assets, Net Carrying Amount	5,096	6,072
Success/Incentive Fees [Member]
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, Gross Cost	35,295	35,422
Other intangible assets, accumulated amortization	31,136	29,723
Other intangible assets, Net Carrying Amount	4,159	5,699
Management Fees, Customer Relationships and Non-Compete Agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, Gross Cost	34,482	33,749
Other intangible assets, accumulated amortization	33,545	33,376
Other intangible assets, Net Carrying Amount	$ 937	$ 373