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Property, Plant, and Equipment (Narative) (Details) - USD ($)
$ in Millions
1 Months Ended 3 Months Ended 12 Months Ended
Jan. 31, 2019
Mar. 31, 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   $ 11    
Impact of adoption of new accounting principle   11    
Accounting Standards Update 2018-11 [Member]        
Property, Plant and Equipment [Line Items]        
Impact of adoption of new accounting principle   $ 15