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Property, Plant, and Equipment (Narative) (Details) - USD ($)
$ in Millions
1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
Jan. 31, 2019
Mar. 31, 2019
Jun. 30, 2019
Dec. 31, 2018
Dec. 31, 2017
Property, Plant and Equipment [Line Items]          
Proceeds from sale of property       $ 106 $ 106
Towers [Member]          
Property, Plant and Equipment [Line Items]          
Proceeds from sale of property $ 76        
Aggregate carrying value 1        
Gain on sale of property $ 75        
Accounting Standards Update 2016-02 [Member]          
Property, Plant and Equipment [Line Items]          
Cumulative-effect adjustment, Net of tax     $ 15    
Cumulative-effect adjustment   $ 11 11    
Accounting Standards Update 2018-11 [Member]          
Property, Plant and Equipment [Line Items]          
Cumulative-effect adjustment, Net of tax     15    
Cumulative-effect adjustment     $ 11