Employee Pension and Other Postretirement Benefits - AOCI Loss for Pension (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss, Gross	$ 916	$ 1,036
Prior service costs (credits), Gross	(100)	(126)
Pension and other benefits adjustment, Gross	(94)	910
Net actuarial loss in AOCI, Tax	241	273
Prior service costs (credits) in AOCI, Tax	(26)	(33)
Pension and other benefits adjustment, Tax	215	240
Net Actuarial (Gain)/Loss, Net of Tax	675	763
Prior service costs (credits), Net of Tax	(74)	(93)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, Net Of Tax	816
Amounts to be reclassified from accumulated other comprehensive loss	601	670
Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss, Gross	33	64
Prior service costs (credits), Gross	0	(14)
Pension and other benefits adjustment, Gross	33	50
Net actuarial loss in AOCI, Tax	9	18
Prior service costs (credits) in AOCI, Tax	0	(4)
Pension and other benefits adjustment, Tax	9	14
Net Actuarial (Gain)/Loss, Net of Tax	24	46
Prior service costs (credits), Net of Tax	0	(10)
Amounts to be reclassified from accumulated other comprehensive loss	24	36
Executive Supplemental Benefits Pension Plan
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss, Gross	39	32
Prior service costs (credits), Gross	0	0
Pension and other benefits adjustment, Gross	39	32
Net actuarial loss in AOCI, Tax	10	8
Prior service costs (credits) in AOCI, Tax	0	0
Pension and other benefits adjustment, Tax	10	8
Net Actuarial (Gain)/Loss, Net of Tax	29	24
Prior service costs (credits), Net of Tax	0	0
Amounts to be reclassified from accumulated other comprehensive loss	$ 29	$ 24