Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 96,639	$ 75,491	$ 185,212	$ 152,262
Long-lived assets	31,495		31,495		$ 30,897
United States [Member]
Segment Reporting Information [Line Items]
Revenues	82,144	65,937	157,518	132,390
Long-lived assets	23,296		23,296		21,968
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	14,027	9,200	26,732	19,264
Long-lived assets	8,176		8,176		8,910
Other [Member]
Segment Reporting Information [Line Items]
Revenues	468	$ 354	962	$ 608
Long-lived assets	$ 23		$ 23		$ 19