Employee Pension and Other Postretirement Benefits - AOCI Loss for Pension (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss, Gross	$ 1,036	$ 905
Prior service costs (credits), Gross	(126)	(152)
Pension and other benefits adjustment, Gross	910	753
Net actuarial loss in AOCI, Tax	273	238
Prior service costs (credits) in AOCI, Tax	(33)	(40)
Pension and other benefits adjustment, Tax	240	198
Net Actuarial (Gain)/Loss, Net of Tax	763	667
Prior service costs (credits), Net of Tax	(93)	(112)
Amounts to be reclassified from accumulated other comprehensive loss	670	555
Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss, Gross	64	51
Prior service costs (credits), Gross	(14)	(29)
Pension and other benefits adjustment, Gross	50	22
Net actuarial loss in AOCI, Tax	18	14
Prior service costs (credits) in AOCI, Tax	(4)	(8)
Pension and other benefits adjustment, Tax	14	6
Net Actuarial (Gain)/Loss, Net of Tax	46	37
Prior service costs (credits), Net of Tax	(10)	(21)
Amounts to be reclassified from accumulated other comprehensive loss	36	16
Executive Supplemental Benefits Pension Plan
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss, Gross	32	40
Prior service costs (credits), Gross	0	0
Pension and other benefits adjustment, Gross	32	40
Net actuarial loss in AOCI, Tax	8	11
Prior service costs (credits) in AOCI, Tax	0	0
Pension and other benefits adjustment, Tax	8	11
Net Actuarial (Gain)/Loss, Net of Tax	24	29
Prior service costs (credits), Net of Tax	0	0
Amounts to be reclassified from accumulated other comprehensive loss	$ 24	$ 29