Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 511,352	$ 435,565	$ 393,422
Long-lived assets	71,795	63,010
United States [Member]
Segment Reporting Information [Line Items]
Revenues	424,731	367,373	332,982
Long-lived assets	62,356	55,200
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	84,076	66,241	58,991
Long-lived assets	9,410	7,787
Other [Member]
Segment Reporting Information [Line Items]
Revenues	2,545	1,951	$ 1,449
Long-lived assets	$ 29	$ 23