Balance Sheet Components - Schedule of Accounts Receivable, Net of allowances (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Valuation And Qualifying Accounts Disclosure [Line Items]
Gross accounts receivable	$ 146,156	$ 138,292
Allowance for accounts receivable	(12,261)	(17,739)
Total Accounts Receivable—net of allowances	133,895	120,553
Allowance for Sales Returns
Valuation And Qualifying Accounts Disclosure [Line Items]
Allowance for accounts receivable	(4,221)	(6,907)	$ (6,916)
Allowance for Sales Incentives
Valuation And Qualifying Accounts Disclosure [Line Items]
Allowance for accounts receivable	(7,271)	(10,442)	$ (8,503)
Other Allowances
Valuation And Qualifying Accounts Disclosure [Line Items]
Allowance for accounts receivable	$ (769)	$ (390)