Goodwill and Other Intangible Assets - Gross Cost and Accumulated Amortization of Other Intangible Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, Gross Cost	$ 69,171	$ 68,987
Other intangible assets, accumulated amortization	63,099	60,080
Other intangible assets, Net Carrying Amount	6,072	8,907
Success/Incentive Fees [Member]
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, Gross Cost	35,422	35,259
Other intangible assets, accumulated amortization	29,723	26,984
Other intangible assets, Net Carrying Amount	5,699	8,275
Management Fees, Customer Relationships and Non-Compete Agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, Gross Cost	33,749	33,728
Other intangible assets, accumulated amortization	33,376	33,096
Other intangible assets, Net Carrying Amount	$ 373	$ 632