Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 124,491	$ 114,714
Long-lived assets	61,047		$ 63,010
United States [Member]
Segment Reporting Information [Line Items]
Revenues	103,615	96,451
Long-lived assets	53,518		55,200
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	20,267	17,751
Long-lived assets	7,508		7,787
Other [Member]
Segment Reporting Information [Line Items]
Revenues	609	$ 512
Long-lived assets	$ 21		$ 23