Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 97,314	$ 96,639	$ 201,205	$ 185,212
Long-lived assets	34,643		34,643		$ 31,104
United States [Member]
Segment Reporting Information [Line Items]
Revenues	82,654	82,136	170,409	157,507
Long-lived assets	25,694		25,694		23,370
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	14,268	14,027	30,027	26,732
Long-lived assets	8,931		8,931		7,713
Other [Member]
Segment Reporting Information [Line Items]
Revenues	392	$ 476	769	$ 973
Long-lived assets	$ 18		$ 18		$ 21