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Financial Statement 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 Receivable [Roll Forward]      
Revenue $ 162,955 $ 152,135 $ 137,801
Carrier Access Billing (CAB)      
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Billings deemed not probable of collection 10,024 10,494 7,163
Billed 19,147 19,838 15,617
Revenue 9,123 9,344 8,454
Accounts Receivable, Excluding Carrier Access Billing (CAB)      
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Balance, beginning of period 255 238  
Billings deemed not probable of collection 176 238  
Deductions (242) (221)  
Balance, end of period 189 255 238
Carrier Access Billing (CAB)      
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Balance, beginning of period 22,316 12,317  
Billings deemed not probable of collection 10,024 10,494  
Deductions (66) (495)  
Balance, end of period $ 32,274 $ 22,316 $ 12,317