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Wireless Licenses, Goodwill and Other Intangible Assets - Additional Information (Detail) - USD ($)
$ in Billions
1 Months Ended 6 Months Ended
Feb. 28, 2018
Jan. 31, 2018
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Apr. 30, 2017
Wireless Licenses            
Indefinite-lived Intangible Assets [Line Items]            
Wireless licenses under development     $ 11.5   $ 8.8  
Interest costs capitalized     $ 0.3 $ 0.2    
Average remaining renewal period of wireless license portfolio (in years)     4 years 6 months      
Straight Path Communications Inc. | Wireless Licenses            
Indefinite-lived Intangible Assets [Line Items]            
Indefinite-lived intangible assets acquired $ 4.5          
XO Holdings            
Indefinite-lived Intangible Assets [Line Items]            
Business acquisition, other intangible assets acquired           $ 0.2
XO Holdings | Wireless Licenses            
Indefinite-lived Intangible Assets [Line Items]            
Indefinite-lived intangible assets acquired   $ 0.7        
Selling, general and administrative expense            
Indefinite-lived Intangible Assets [Line Items]            
Goodwill impairment charge     $ 0.1