Stockholders' Equity - Net Income Attributable to Noncontrolling Interests (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change In Ownership Interest [Line Items]
Net Income Attributable to Noncontrolling Interests	$ 604	$ 2,260	$ 523	$ 2,877	$ 1,005	$ 82	$ 1,007	$ 3,900	$ 6,264	$ 5,994	$ 6,559
Noncontrolling interests	59,102				57,826				59,102	57,826
Edgewater [Member]
Change In Ownership Interest [Line Items]
Net Income Attributable to Noncontrolling Interests									6,259	5,992	6,557
Noncontrolling interests	58,568				57,238				58,568	57,238
Other [Member]
Change In Ownership Interest [Line Items]
Net Income Attributable to Noncontrolling Interests									5	2	$ 2
Noncontrolling interests	$ 534				$ 588				$ 534	$ 588