Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 125,490	$ 106,964	$ 249,981	$ 221,678
Long-lived assets	64,206		64,206		$ 63,010
United States [Member]
Segment Reporting Information [Line Items]
Revenues	104,141	90,272	207,756	186,723
Long-lived assets	57,121		57,121		55,200
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	20,706	16,195	40,973	33,946
Long-lived assets	7,050		7,050		7,787
Other [Member]
Segment Reporting Information [Line Items]
Revenues	643	$ 497	1,252	$ 1,009
Long-lived assets	$ 35		$ 35		$ 23