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Benefit Plans Contributions (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Sep. 30, 2016
Sep. 30, 2015
Other Pension Plans, Defined Benefit        
Defined Benefit Plan Disclosure        
Defined Benefit Plan, Contributions by Employer       $ 275
Other Post-retirement Benefits        
Defined Benefit Plan Disclosure        
Defined Benefit Plan, Contributions by Employer $ 17 $ 16    
Scenario, Forecast | Other Pension Plans, Defined Benefit        
Defined Benefit Plan Disclosure        
Defined Benefit Plan, Contributions by Employer     $ 275