Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 106,964	$ 96,314	$ 221,678	$ 199,278
Long-lived assets	50,088		50,088		$ 38,548
United States [Member]
Segment Reporting Information [Line Items]
Revenues	90,272	81,682	186,723	168,539
Long-lived assets	40,968		40,968		27,990
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	16,195	14,240	33,946	29,971
Long-lived assets	9,097		9,097		10,532
Other [Member]
Segment Reporting Information [Line Items]
Revenues	497	$ 392	1,009	$ 768
Long-lived assets	$ 23		$ 23		$ 26