Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 397,471	$ 369,919	$ 303,098
Long-lived assets	38,548	31,104
United States [Member]
Segment Reporting Information [Line Items]
Revenues	336,867	314,343	262,558
Long-lived assets	27,990	23,370
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	59,151	54,015	39,338
Long-lived assets	10,532	7,713
Other [Member]
Segment Reporting Information [Line Items]
Revenues	1,453	1,561	$ 1,202
Long-lived assets	$ 26	$ 21