Commitments and Contingencies - Universal Services Fund Liability (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	Jul. 31, 2013	Mar. 31, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Dec. 31, 2012
Loss Contingencies [Line Items]
Accrued federal fees			$ 1,151	$ 2,742
Increase in accrued federal fees			0	$ (3,114)	$ 0
Promissory note payable			32,927
Year 2003 through 2012 \| USF Obligations
Loss Contingencies [Line Items]
Accrued federal fees							$ 8,100
Undisputed portion \| Year 2003 through 2012 \| USF Obligations
Loss Contingencies [Line Items]
Accrued federal fees							4,700
Payments on usage-based fees	$ 600
Disputed portion \| Year 2003 through 2012 \| USF Obligations
Loss Contingencies [Line Items]
Accrued federal fees		$ 3,900					$ 3,400
USF credits						$ 3,100
Disputed liability related to reversal of interest and penalties						3,100
Disputed portion \| Year 2008 Through 2012 \| USF Obligations
Loss Contingencies [Line Items]
Increase in accrued federal fees		$ 500
Disputed portion \| Year 2003 To Year 2007 \| USF Obligations
Loss Contingencies [Line Items]
Accrued federal fees			800			$ 800
Promissory Note \| USF Obligations
Loss Contingencies [Line Items]
Promissory note payable	$ 4,100
Universal Services Fund
Loss Contingencies [Line Items]
USF credits			$ 700