Balance Sheet Components - Schedule of Accounts Receivable, Net of Allowances (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Valuation And Qualifying Accounts Disclosure [Line Items]
Gross accounts receivable	$ 209,149	$ 204,975
Allowance for accounts receivable	(13,105)	(21,897)
Total Accounts Receivable—net of allowances	196,044	183,078
Allowance for Sales Returns
Valuation And Qualifying Accounts Disclosure [Line Items]
Allowance for accounts receivable	(4,438)	(7,335)	$ (6,907)
Allowance for Sales Incentives
Valuation And Qualifying Accounts Disclosure [Line Items]
Allowance for accounts receivable	(8,378)	(13,750)	$ (10,442)
Other Allowances
Valuation And Qualifying Accounts Disclosure [Line Items]
Allowance for accounts receivable	$ (289)	$ (812)