Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 435,565	$ 393,422	$ 367,730
Long-lived assets	63,010	38,548
United States [Member]
Segment Reporting Information [Line Items]
Revenues	367,373	332,982	312,304
Long-lived assets	55,200	27,990
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	66,241	58,991	53,865
Long-lived assets	7,787	10,532
Other [Member]
Segment Reporting Information [Line Items]
Revenues	1,951	1,449	$ 1,561
Long-lived assets	$ 23	$ 26