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Employee Benefit Plans - Net Periodic Benefits Cost and Other Recognized in the OCI (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension Benefits      
Defined Benefit Plan, Net Periodic Cost (Benefit)      
Service cost $ 4 $ 2 $ 2
Interest cost 170 237 235
Expected return on plan assets (267) (311) (311)
Amortization of actuarial loss 56 56 60
Settlements 750    
Net periodic cost (benefit) 713 (16) (14)
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss)      
Amortization of actuarial loss 56 56  
Settlement loss 750 0  
Amortization of prior service credit 0 0  
Net loss arising during the year (209) (66)  
Total 597 (10)  
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]      
Net loss (1,966) (2,563)  
Prior service credit 0 0  
Total (1,966) (2,563)  
Other Postretirement Benefits      
Defined Benefit Plan, Net Periodic Cost (Benefit)      
Service cost 0 0  
Interest cost 8 11 12
Expected return on plan assets (8) (10) (12)
Amortization of prior service credit (7) (6) (7)
Amortization of actuarial loss 5 5 5
Settlements   0  
Net periodic cost (benefit) (2) 0 $ (2)
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss)      
Amortization of actuarial loss 5 5  
Settlement loss 0 0  
Amortization of prior service credit (7) (6)  
Net loss arising during the year (12) (4)  
Total (14) (5)  
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]      
Net loss (129) (122)  
Prior service credit 78 85  
Total $ (51) $ (37)