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Benefit Plans Contributions (Details) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Jun. 30, 2018
Jun. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Retirement Plan Disclosure          
401(k) Contributions   $ 61,000,000      
Other Pension Plan          
Retirement Plan Disclosure          
Defined benefit plan contributions   225,000,000     $ 800,000,000
Other Post-retirement Benefits          
Retirement Plan Disclosure          
Defined benefit plan contributions   17,000,000 $ 22,000,000    
Scenario, Forecast | Other Pension Plan          
Retirement Plan Disclosure          
Defined benefit plan contributions $ 75,000,000     $ 300,000,000  
Minimum | Other Pension Plan          
Retirement Plan Disclosure          
Defined benefit plan contributions   $ 300,000,000