Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Revenues	$ 369,919	$ 303,098	$ 262,774
Long-lived assets	31,104	30,897
United States [Member]
Segment Reporting Information [Line Items]
Revenues	314,343	262,558	223,741
Long-lived assets	23,370	21,968
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	54,015	39,338	38,189
Long-lived assets	7,713	8,910
Other [Member]
Segment Reporting Information [Line Items]
Revenues	1,561	1,202	$ 844
Long-lived assets	$ 21	$ 19