Industry and Geographic Information - Long-lived Assets (Excluding Intangible Assets) and Total Net Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets, Total	$ 61,585				$ 50,858				$ 61,585	$ 50,858
Total revenues	114,890	$ 50,894	$ 38,267	$ 73,692	52,808	$ 49,341	$ 39,133	$ 50,321	277,743	191,603	$ 196,129
Federal [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets, Total	59,833				50,774				59,833	50,774
Total revenues									227,611	158,244	168,809
Foreign [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets, Total	$ 1,752				$ 84				1,752	84
Total revenues									$ 50,132	$ 33,359	$ 27,320