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Balance Sheet Components - Allowance for Doubtful Accounts and Reserve for Subscriber Refunds (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Allowance for doubtful accounts      
Movement in Valuation Allowances and Reserves [Roll Forward]      
Balance at the beginning of the year $ 295 $ 384 $ 444
Additions — charged to costs and expenses, or contra revenue, net 158 107 295
Deductions — recoveries of amounts previously charged-off (125) (89) (179)
Deductions — write-offs (13) (107) (176)
Balance at end of year 315 295 384
Reserve for member refunds      
Movement in Valuation Allowances and Reserves [Roll Forward]      
Balance at the beginning of the year 474 530 799
Additions — charged to costs and expenses, or contra revenue, net 942 507 776
Deductions — recoveries of amounts previously charged-off 0 0 0
Deductions — write-offs (886) (563) (1,045)
Balance at end of year $ 530 $ 474 $ 530