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Accounting Policies - Narrative (Details) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2017
Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Reclassification of cash flows from Operating Activities $ (154) $ 290    
Reclassification of cash flows to Financing Activities (457) 109    
Restricted cash balances $ 46 25   $ 57
Scenario, Forecast        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Non-service costs     $ 70  
Accounting Standards Update 2016-15 | New Accounting Pronouncement, Early Adoption, Effect        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Reclassification of cash flows from Operating Activities   53    
Reclassification of cash flows to Financing Activities   $ 53