Segment Reporting and Geographic Locations - Revenue and Long-Lived Assets for Each of Geographic Areas (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]
Revenues	$ 223,602	$ 225,010	$ 233,391	$ 215,734	$ 188,339	$ 198,349	$ 195,127	$ 173,416	$ 897,737	$ 755,231	$ 662,112
Long-lived Assets	65,099				64,630				65,099	64,630	66,041
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									670,075	560,412	496,473
Long-lived Assets	50,872				50,207				50,872	50,207	54,334
All other countries
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									227,662	194,819	165,639
Long-lived Assets	$ 14,227				$ 14,423				$ 14,227	$ 14,423	$ 11,707