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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, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Benefits      
Defined Benefit Plan, Net Periodic Cost (Benefit)      
Service cost $ (4) $ (4) $ (4)
Interest cost 127 159 142
Expected return on plan assets (215) (226) (227)
Amortization of prior service credit 0 0 0
Amortization of actuarial loss 60 44 49
Net periodic cost (benefit) (24) (19) (32)
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss)      
Amortization of actuarial loss 60 44 49
Amortization of prior service credit 0 0 0
Prior service cost (credit) (106) (88) (91)
Prior service cost 0 0 0
Total (46) (44) (42)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]      
Net loss (2,098) (2,052) (2,008)
Prior service credit 0 0 0
Total (2,098) (2,052) (2,008)
Other Postretirement Benefits      
Defined Benefit Plan, Net Periodic Cost (Benefit)      
Service cost 0 0 0
Interest cost 6 8 7
Expected return on plan assets (4) (4) (7)
Amortization of prior service credit (7) (7) (7)
Amortization of actuarial loss 7 6 6
Net periodic cost (benefit) 2 3 (1)
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss)      
Amortization of actuarial loss 7 6 6
Amortization of prior service credit (7) (7) (6)
Prior service cost (credit) (11) (18) 3
Prior service cost 0 2 0
Total (11) (17) 3
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]      
Net loss (136) (132) (120)
Prior service credit 60 67 72
Total $ (76) $ (65) $ (48)