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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets, Total	$ 73,901				$ 61,585				$ 73,901	$ 61,585
Revenue from Contract with Customer, Including Assessed Tax	132,588	$ 117,399	$ 103,155	$ 169,143	114,890	$ 50,894	$ 38,267	$ 73,692	522,285	277,743	$ 191,603
Federal [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets, Total	72,569				59,833				72,569	59,833
Revenue from Contract with Customer, Including Assessed Tax									354,895	227,611	158,244
Foreign [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets, Total	$ 1,332				$ 1,752				1,332	1,752
Revenue from Contract with Customer, Including Assessed Tax									$ 167,390	$ 50,132	$ 33,359