Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 96,701	$ 90,271	$ 297,906	$ 275,483
Long-lived assets	35,321		35,321		$ 31,104
United States [Member]
Segment Reporting Information [Line Items]
Revenues	82,076	77,571	252,485	235,078
Long-lived assets	25,602		25,602		23,370
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	14,250	12,433	44,277	39,165
Long-lived assets	9,700		9,700		7,713
Other [Member]
Segment Reporting Information [Line Items]
Revenues	375	$ 267	1,144	$ 1,240
Long-lived assets	$ 19		$ 19		$ 21