Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 114,714	$ 102,964
Long-lived assets	39,623		$ 38,548
United States [Member]
Segment Reporting Information [Line Items]
Revenues	96,451	86,856
Long-lived assets	29,176		27,990
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	17,751	15,732
Long-lived assets	10,422		10,532
Other [Member]
Segment Reporting Information [Line Items]
Revenues	512	$ 376
Long-lived assets	$ 25		$ 26