Income Taxes - Schedule of Deferred Income Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets
Net operating loss and credit carry-forwards	$ 73,016	$ 59,863	$ 54,858
Research and development credits	8,027	6,094	4,712
Sales tax liability	627	1,131	1,337
Share-based compensation	4,955	7,281	6,694
Accrued liabilities	4,881	6,525	6,090
Gross deferred tax assets	91,506	80,894	73,691
Valuation allowance	(89,235)	(79,319)	(71,514)
Total deferred tax assets	2,271	1,575	2,177
Deferred tax liabilities - Acquired intangibles	(344)	(803)	(1,164)
Deferred tax liabilities - Property and equipment	(1,714)	(606)	(883)
Net deferred tax assets	$ 213	$ 166	$ 130