Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 131,597	$ 101,443	$ 381,578	$ 323,121
Long-lived assets	66,531		66,531		$ 63,010
United States [Member]
Segment Reporting Information [Line Items]
Revenues	109,526	85,110	317,282	271,833
Long-lived assets	59,146		59,146		55,200
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	21,417	15,882	62,390	49,828
Long-lived assets	7,355		7,355		7,787
Other [Member]
Segment Reporting Information [Line Items]
Revenues	654	$ 451	1,906	$ 1,460
Long-lived assets	$ 30		$ 30		$ 23