Segment and Geographic Information - Summary of Revenue and Long-lived Assets (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Revenue	$ 262,774	$ 238,733	$ 190,840
Long-lived assets	32,185	32,703
United States [Member]
Segment Reporting Information [Line Items]
Revenue	223,741	207,519	177,280
Long-lived assets	23,099	24,382
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenue	38,189	30,722	13,344
Long-lived assets	9,057	8,305
Other [Member]
Segment Reporting Information [Line Items]
Revenue	844	492	216
Long-lived assets	$ 29	$ 17