Goodwill and Other Intangible Assets - Gross Cost and Accumulated Amortization of Other Intangible Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, Gross Cost	$ 69,823	$ 69,171
Other intangible assets, accumulated amortization	65,576	63,099
Other intangible assets, Net Carrying Amount	4,247	6,072
Success/Incentive Fees [Member]
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, Gross Cost	35,335	35,422
Other intangible assets, accumulated amortization	31,922	29,723
Other intangible assets, Net Carrying Amount	3,413	5,699
Management Fees, Customer Relationships and Non-Compete Agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, Gross Cost	34,488	33,749
Other intangible assets, accumulated amortization	33,654	33,376
Other intangible assets, Net Carrying Amount	$ 834	$ 373