Goodwill and Other Intangible Assets - Gross Cost and Accumulated Amortization of Other Intangible Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, Gross Cost	$ 70,236	$ 70,126
Other intangible assets, accumulated amortization	69,590	68,305
Other intangible assets, Net Carrying Amount	646	1,821
Success/Incentive Fees [Member]
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, Gross Cost	35,271	35,338
Other intangible assets, accumulated amortization	34,896	33,840
Other intangible assets, Net Carrying Amount	375	1,498
Management Fees, Customer Relationships and Non-Compete Agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, Gross Cost	34,965	34,788
Other intangible assets, accumulated amortization	34,694	34,465
Other intangible assets, Net Carrying Amount	$ 271	$ 323