Balance Sheet Components - Summary of Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash and Cash Equivalents [Abstract]
Cash	$ 556	$ 1,964
Cash equivalents:
Money market accounts	99,792	107,043
Total cash and cash equivalents	$ 100,348	$ 109,007	$ 150,180	$ 29,154