Employee Pension and Other Postretirement Benefits - Periodic Benefit Costs, Forecasted (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plan and Other Postretirement Benefits Plan
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		$ 16,367	$ 16,580	$ 18,355	$ 17,386
Amortization period of net actuarial losses		8 years
Excess of market value of plan assets		$ 264
Assetsmoothingperiodofnonamortizing amount		4 years
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss, Gross		$ (916)	$ (1,036)
Actuarial loss, subject to amortization		$ 792
Expected return on plan assets		7.00%	8.00%
Rate of increase in future compensation levels for determining net periodic benefit cost		5.00%	5.00%
Net periodic benefit cost		$ (29)	$ (27)	$ (31)
Prior service costs (credits), Gross		$ (100)	$ (126)
Pension Benefits | Forecast
Pension Plan and Other Postretirement Benefits Plan
Assumptions used to calculate net periodic benefit cost, service cost rate	3.16%
Defined benefit plan, assumptions used calculating net periodic benefit cost, interest cost	2.72%
Expected return on plan assets	7.00%
Rate of increase in future compensation levels for determining net periodic benefit cost	5.10%
Net periodic benefit cost	$ 5
Effect of 50 basis point increase on discount rate	28
Effect of 50 basis point increase on expected rate of return	19
Effect of 50 basis point increase on future compensation levels	2
Effect of 50 basis point decrease on discount rate	31
Effect of 50 basis point decrease on expected rate of return on plan assets	19
Effect of 50 basis point decrease on future compensation levels	$ 2