Annual report pursuant to Section 13 and 15(d)

Commitments and Contingencies (Tribal Mobility Fund I Grant) (Details)

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Commitments and Contingencies (Tribal Mobility Fund I Grant) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Feb. 28, 2014
Other Commitments [Line Items]        
Grant proceeds $ 2,188 $ 1,527 $ 14,007  
FCC        
Other Commitments [Line Items]        
Federal grant award       $ 41,400
Grant proceeds 16,800 $ 0 $ 13,800  
Grant awards $ 10,800