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Commitments and Contingencies - Universal Services Fund Liability (Details) - USD ($)
$ in Thousands
1 Months Ended 3 Months Ended
Jul. 31, 2013
Mar. 31, 2013
Jun. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2012
Loss Contingencies [Line Items]            
Accrued federal fees     $ 1,638 $ 1,151    
Principal     258,781      
2003 through 2012 | USF obligations            
Loss Contingencies [Line Items]            
Accrued federal fees           $ 8,100
2003 through 2012 | Undisputed portion | USF obligations            
Loss Contingencies [Line Items]            
Accrued federal fees           4,700
Payments $ 600          
2003 through 2012 | Disputed portion | USF obligations            
Loss Contingencies [Line Items]            
Accrued federal fees   $ 3,900       $ 3,400
Disputed liability related to the reversal of interest and penalties         $ 3,100  
USF credits         3,100  
Year 2003 To Year 2007 | Disputed portion | USF obligations            
Loss Contingencies [Line Items]            
Accrued federal fees     800   $ 800  
2008 through 2012 | Disputed portion | USF obligations            
Loss Contingencies [Line Items]            
Increase (decrease) in accrual federal fees   $ 500        
Promissory Note | USF obligations            
Loss Contingencies [Line Items]            
Principal $ 4,100          
Universal Services Fund            
Loss Contingencies [Line Items]            
USF credits     $ 700