Employee Pension and Other Postretirement Benefits - Weighted Average Assumptions, PV of Benefit Obligations and Net Periodic Benefit Cost (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
Discount rate in determining net periodic benefit cost after Plan amendments, interest cost		4.00%	3.00%
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Discount rate in determining net periodic benefit cost after Plan amendments, service cost		4.00%	3.00%
Discount rate in determining net periodic benefit cost after Plan amendments, interest cost		4.00%	3.00%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate		3.00%	4.00%	3.00%
Rate of increase in future compensation levels for determining net periodic benefit cost		5.00%	5.00%
Rate of increase in future compensation levels for determining benefit obligations at year end		5.00%	5.00%
Expected return on plan assets		7.00%	8.00%
Service cost		$ 76	$ 80	$ 71
Interest cost		117	100	100
Expected return on plan assets		(257)	(268)	(254)
Amortization of prior service credit		(26)	(26)	(27)
Recognized net actuarial loss		61	87	79
Curtailment gain		0	0	0
Total net periodic benefit cost		$ (29)	$ (27)	$ (31)
Defined benefit plan assumptions used calculating net periodic benefit cost cash balance crediting rate		3.00%	3.00%
Defined benefit plan assumptions used calculating benefit obligations cash balance crediting rate		2.00%	3.00%
Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Discount rate in determining net periodic benefit cost after Plan amendments, service cost		4.00%	3.00%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate		3.00%	4.00%	3.00%
Expected return on plan assets		7.00%	8.00%
Service cost		$ 3	$ 4	$ 6
Interest cost		9	8	8
Expected return on plan assets		(17)	(21)	(19)
Amortization of prior service credit		(14)	(15)	(21)
Recognized net actuarial loss		9	5	8
Curtailment gain		0	0	0
Total net periodic benefit cost		(10)	(19)	(18)
Superannuation, SERP and ESBP
Pension Plan and Other Postretirement Benefits Plan
Service cost		0	0	0
Interest cost		3	3	3
Expected return on plan assets		0	0	0
Amortization of prior service credit		0	0	0
Recognized net actuarial loss		4	3	4
Curtailment gain		0	0	0
Total net periodic benefit cost		$ 7	$ 6	$ 7
Forecast | Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Rate of increase in future compensation levels for determining net periodic benefit cost	5.10%
Expected return on plan assets	7.00%
Total net periodic benefit cost	$ 5