Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 103,891	$ 88,573
Long-lived assets	33,888		$ 31,104
United States [Member]
Segment Reporting Information [Line Items]
Revenues	87,755	75,372
Long-lived assets	26,014		23,370
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	15,759	12,704
Long-lived assets	7,854		7,713
Other [Member]
Segment Reporting Information [Line Items]
Revenues	377	$ 497
Long-lived assets	$ 20		$ 21