Stockholders' Equity - Net Income Attributable to Noncontrolling Interests (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Change In Ownership Interest [Line Items]
Net Income Attributable to Noncontrolling Interests	$ 1,492	$ 1,651	$ 8,662	$ 5,036
Noncontrolling interests	72,779		72,779		$ 53,242
Profits Interest Participation Rights [Member]
Change In Ownership Interest [Line Items]
Noncontrolling interests	7,343		7,343		535
Consolidated VIEs [Member]
Change In Ownership Interest [Line Items]
Net Income Attributable to Noncontrolling Interests	189		423
Noncontrolling interests	13,110		13,110
Edgewater [Member]
Change In Ownership Interest [Line Items]
Net Income Attributable to Noncontrolling Interests	1,302	$ 1,651	8,237	5,034
Noncontrolling interests	52,311		52,311		52,695
Other [Member]
Change In Ownership Interest [Line Items]
Net Income Attributable to Noncontrolling Interests	1		2	$ 2
Noncontrolling interests	$ 15		$ 15		$ 12