Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Revenues	$ 74,192	$ 64,245	$ 226,454	$ 192,607
Long-lived assets	31,027		31,027		$ 32,185
United States [Member]
Segment Reporting Information [Line Items]
Revenues	64,098	54,849	196,488	163,469
Long-lived assets	21,868		21,868		23,099
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	9,830	9,197	29,094	28,556
Long-lived assets	9,140		9,140		9,057
Other [Member]
Segment Reporting Information [Line Items]
Revenues	264	$ 199	872	$ 582
Long-lived assets	$ 19		$ 19		$ 29