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Employee Benefit Plans, Pre-Tax Amounts in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Pension Plans [Member]    
Pension and Other Postretirement Benefit Plans Accumulated Other Comprehensive Income (Loss), before Tax    
Net actuarial (gain) loss $ 894 $ 878
Prior service credit (121) (145)
Total 773 733
Other Postretirement Benefit Plans [Member]    
Pension and Other Postretirement Benefit Plans Accumulated Other Comprehensive Income (Loss), before Tax    
Net actuarial (gain) loss (57) (66)
Prior service credit (42) (58)
Total $ (99) $ (124)