Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 90,271	$ 74,192	$ 275,483	$ 226,454
Long-lived assets	30,773		30,773		$ 30,897
United States [Member]
Segment Reporting Information [Line Items]
Revenues	77,571	64,098	235,078	196,488
Long-lived assets	22,674		22,674		21,968
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	12,433	9,830	39,165	29,094
Long-lived assets	8,077		8,077		8,910
Other [Member]
Segment Reporting Information [Line Items]
Revenues	267	$ 264	1,240	$ 872
Long-lived assets	$ 22		$ 22		$ 19