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Derivatives (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Derivative          
Expected payments for derivative instruments $ 725        
Cash receipts associated withe the hedge deferral 11        
Gains on cash flow hedge contracts in AOCI to be reclassified into earnings within 12 months 26        
Margin posted due to fair value position of hedge contracts 454     $ 119  
Fuel Derivative          
Derivative          
Expected proceeds from derivative instruments       $ 300  
Expected payments for derivative instruments 300        
Scenario, Forecast          
Derivative          
Expected proceeds from derivative instruments     $ 300    
Expected payments for derivative instruments         $ 300
Fuel hedge contracts          
Derivative          
Derivative loss $ 274 $ 411