Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Revenues	$ 164,006	$ 131,597	$ 517,779	$ 381,578
Long-lived assets	82,074		82,074		$ 71,795
United States [Member]
Segment Reporting Information [Line Items]
Revenues	138,134	109,526	437,471	317,282
Long-lived assets	65,357		65,357		62,356
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	20,556	21,417	65,061	62,390
Long-lived assets	16,700		16,700		9,410
Other [Member]
Segment Reporting Information [Line Items]
Revenues	5,316	$ 654	15,247	$ 1,906
Long-lived assets	$ 17		$ 17		$ 29