Balance Sheet Components - Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for doubtful accounts
Beginning balance	$ 2,000	$ 1,890	$ 971
Provision for estimated bad debts	(41)	143	984
Write offs	(171)	(33)	(65)
Ending balance	$ 1,788	$ 2,000	$ 1,890