Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 101,443	$ 95,584	$ 323,121	$ 294,862
Long-lived assets	54,320		54,320		$ 38,548
United States [Member]
Segment Reporting Information [Line Items]
Revenues	85,110	80,999	271,833	249,538
Long-lived assets	45,734		45,734		27,990
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	15,882	14,210	49,828	44,181
Long-lived assets	8,564		8,564		10,532
Other [Member]
Segment Reporting Information [Line Items]
Revenues	451	$ 375	1,460	$ 1,143
Long-lived assets	$ 22		$ 22		$ 26