Income Taxes - Differences in Total Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Tax benefit at U.S. statutory rate	$ (10,365)	$ (5,126)	$ (10,997)
State income taxes, net of federal benefit	(2,858)	(147)	(104)
Foreign income and withholding taxes	5,578	4,317	6,033
Impact of change to U.S. statutory rate from Tax Cuts and Jobs Act	14,302	0	0
Stock-based compensation	1,085	2,231	1,039
Change in valuation allowance	(8,158)	155	3,807
Research and development credits	(382)	(736)	(1,067)
Uncertain tax positions	1,995	1,146	1,277
Provision to return adjustments	153	(484)	848
Impact of adoption of ASU 2016-09	(1,291)	0	0
Other	948	48	169
Provision for income taxes	$ 1,007	$ 1,404	$ 1,005