Income Taxes - Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Deferred income tax assets:
Net operating loss carryforwards and credits	$ 14,713	$ 16,090
Acquired intangible assets	4,370	6,561
Reserves and accruals	1,984	2,209
Share-based compensation	1,549	2,837
Depreciation	269	395
Total deferred income tax assets	22,885	28,092
Deferred income tax liabilities:
Acquired intangible assets	(780)	(549)
Other	(379)	(312)
Total deferred income tax liabilities	(1,159)	(861)
Valuation allowance	$ (21,726)	(8,200)	$ (4,500)	$ (1,500)
Deferred income tax asset, net		$ 19,031