Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Revenues	$ 303,098	$ 262,774	$ 238,733
Long-lived assets	30,897	32,185
United States [Member]
Segment Reporting Information [Line Items]
Revenues	262,558	223,741	207,519
Long-lived assets	21,968	23,099
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	39,338	38,189	30,722
Long-lived assets	8,910	9,057
Other [Member]
Segment Reporting Information [Line Items]
Revenues	1,202	844	$ 492
Long-lived assets	$ 19	$ 29