Balance Sheet Components - Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for doubtful accounts
Beginning balance	$ 1,890	$ 971	$ 527
Provision for estimated bad debts	143	984	529
Write offs	(33)	(65)	(85)
Ending balance	$ 2,000	$ 1,890	$ 971