Employee Benefit Plans - Net Periodic Benefits Cost and Other Recognized in the OCI (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Defined Benefit Plan, Net Periodic Cost (Benefit)
Service cost	$ (4)	$ (4)	$ (4)
Interest cost	159	142	170
Expected return on plan assets	(226)	(227)	(267)
Amortization of prior service credit	0	0
Amortization of actuarial loss	44	49	56
Settlements	0	0	750
Net periodic cost (benefit)	(19)	(32)	713
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss)
Amortization of actuarial loss	44	49	56
Settlement loss	0	0	750
Amortization of prior service credit	0	0	0
Prior service cost (credit)	(88)	(91)	(209)
Prior service cost	0	0	0
Total	(44)	(42)	597
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Net loss	(2,052)	(2,008)	(1,966)
Prior service credit	0	0	0
Total	(2,052)	(2,008)	(1,966)
Other Postretirement Benefits
Defined Benefit Plan, Net Periodic Cost (Benefit)
Service cost	0	0
Interest cost	8	7	8
Expected return on plan assets	(4)	(7)	(8)
Amortization of prior service credit	(7)	(7)	(7)
Amortization of actuarial loss	6	6	5
Settlements	0
Net periodic cost (benefit)	3	(1)	(2)
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss)
Amortization of actuarial loss	6	6	5
Settlement loss	0	0	0
Amortization of prior service credit	(7)	(6)	(7)
Prior service cost (credit)	(18)	3	(12)
Prior service cost	2	0	0
Total	(17)	3	(14)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Net loss	(132)	(120)	(129)
Prior service credit	67	72	78
Total	$ (65)	$ (48)	$ (51)