Employee Pension and Other Postretirement Benefits - Pension and Post retirement Benefit Payments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plan and Other Postretirement Benefits Plan
Amortization period of net actuarial losses		8 years
Assetsmoothingperiodofnonamortizing amount		4 years
Pension and Executive Employee Supplemental Benefit, Pension Plan [Member]
Pension Plan and Other Postretirement Benefits Plan
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax		$ 855	$ 942
Defined Benefit Plan, Accumulated Other Comprehensive Income, Tax		225	248
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax		630	694
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax		(94)	910
Defined Benefit Plan, Service Cost		76	80	$ 71
2018		149
2019		156
2020		162
2021		169
2022		173
Years 2023-2027		939
Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year			(107)
Defined Benefit Plan, Interest Cost		117	100	100
Defined Benefit Plan, Expected Return (Loss) on Plan Assets		(257)	(268)	(254)
Amortization of prior service credit		(26)	(26)	(27)
Defined Benefit Plan, Amortization of Gain (Loss)		61	87	79
Curtailment gain		0	0	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		(29)	(27)	(31)
Defined Benefit Plan, Accumulated Other Comprehensive Income, Tax		215	240
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax		601	670
Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax		33	50
Defined Benefit Plan, Service Cost		3	4	6
2018		17
2019		17
2020		18
2021		18
2022		17
Years 2023-2027		72
Defined Benefit Plan, Interest Cost		9	8	8
Defined Benefit Plan, Expected Return (Loss) on Plan Assets		(17)	(21)	(19)
Amortization of prior service credit		(14)	(15)	(21)
Defined Benefit Plan, Amortization of Gain (Loss)		9	5	8
Curtailment gain		0	0	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		(10)	(19)	(18)
Defined Benefit Plan, Accumulated Other Comprehensive Income, Tax		9	14
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax		24	36
Superannuation Supplemental Executives Retirement Plan Defined Benefit and Executive Supplemental Defined Benefit Plan [Member]
Pension Plan and Other Postretirement Benefits Plan
Defined Benefit Plan, Service Cost		0	0	0
Defined Benefit Plan, Interest Cost		3	3	3
Defined Benefit Plan, Expected Return (Loss) on Plan Assets		0	0	0
Amortization of prior service credit		0	0	0
Defined Benefit Plan, Amortization of Gain (Loss)		4	3	4
Curtailment gain		0	0	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		$ 7	$ 6	$ 7
Forecast | Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Effect of 50 basis point increase on discount rate	$ 28
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	$ 5