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Acquisitions and Dispositions - Assets Acquired and Liabilities Assumed (Details) - USD ($)
$ in Millions
Feb. 14, 2017
Mar. 31, 2017
Dec. 31, 2016
Business Acquisition [Line Items]      
Goodwill   $ 2,596 $ 2,344
ACC Acquisition      
Business Acquisition [Line Items]      
Property and equipment $ 299    
Intangible assets 641    
Goodwill 278    
Other (including $4 million of cash acquired) (1)    
Total $ 1,217    
Finite lived intangible assets, useful life 20 years    
Cash acquired $ 4    
Minimum | ACC Acquisition      
Business Acquisition [Line Items]      
Property and equipment, useful life 3 years    
Maximum | ACC Acquisition      
Business Acquisition [Line Items]      
Property and equipment, useful life 70 years