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Commitments and Contingencies (PurchOb) (Details) - USD ($)
$ in Millions
Dec. 31, 2016
Dec. 31, 2015
Letters of credit and surety bonds    
Letters of credit and surety bonds    
Outstanding letters of credit $ 50 $ 153
Aggregate cash collateral 5  
Surety bonds outstanding 33 $ 30
Supply agreement    
Future Purchase obligations    
2017 27  
2018 47  
2019 50  
2020 52  
Thereafter 366  
Total 542  
New builds    
Future Purchase obligations    
2017 229  
2018 9  
2020 1,766  
Total $ 2,004