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, 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]
Revenues	$ 188,339	$ 198,349	$ 195,127	$ 173,416	$ 149,966	$ 153,604	$ 178,251	$ 180,291	$ 755,231	$ 662,112	$ 825,341
Long-lived Assets	64,630				66,041				64,630	66,041	70,961
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									560,412	496,473	635,627
Long-lived Assets	50,207				54,334				50,207	54,334	59,280
All other countries
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									194,819	165,639	189,714
Long-lived Assets	$ 14,423				$ 11,707				$ 14,423	$ 11,707	$ 11,681