Employee Pension and Other Postretirement Benefits - Additional Information (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
Employee pre-tax covered compensation percent		75.00%
Employee after-tax covered compensation percent		10.00%
Combined maximum employee contribution percent		75.00%
Employee pre-tax gross pay for which the employer contributes a match		4.00%
Contribution plan expenses		$ 61	59	55
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Pension and other benefits adjustment, Gross		(94)	910
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, Net Of Tax		$ 816
Service Period required for eligibility		1 year
Service Period required for vesting		3 years
Pension and other postretirement plan investments impacted by ASU 2018-09		$ 4,140	3,529	3,854
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax, Subject to Amortization		(792)
Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year			(107)
Net periodic benefit cost		$ (29)	$ (27)	(31)
Expected return on plan assets		7.00%	8.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase		5.00%	5.00%
Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Pension and other benefits adjustment, Gross		$ 33	$ 50
Pension and other postretirement plan investments impacted by ASU 2018-09		291	255	288
Net periodic benefit cost		$ (10)	$ (19)	(18)
Expected return on plan assets		7.00%	8.00%
Executive Supplemental Benefits Pension Plan
Pension Plan and Other Postretirement Benefits Plan
Accrued liability for ESBPs		$ 94	$ 89
Pension and other benefits adjustment, Gross		$ 39	32
Equity securities | Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation		53.00%
Equity securities | Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation		70.00%
Debt Securities | Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation		35.00%
Debt Securities | Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation		30.00%
Real estate | Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation		12.00%
Minimum | Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Defined Benefit Plan Future Cost Sharing Percentage		25.00%
Maximum | Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Defined Benefit Plan Future Cost Sharing Percentage		50.00%
Recognized net actuarial gain (loss) | Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		$ 916	1,036	905	1,085
Recognized net actuarial gain (loss) | Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		33	64	51	78
Amortization of prior service credit | Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		(100)	(126)	(152)	(179)
Amortization of prior service credit | Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		$ 0	(14)	$ (29)	$ (50)
Forecast | Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Net periodic benefit cost	$ 5
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%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	5.10%
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
Matching Range 1
Pension Plan and Other Postretirement Benefits Plan
Company pre-tax compensation match		100.00%
Employee pre-tax gross pay for which the employer contributes a match		3.00%
Matching Range 2
Pension Plan and Other Postretirement Benefits Plan
Company pre-tax compensation match		50.00%
Employee pre-tax gross pay for which the employer contributes a match		2.00%
Accounting Standards Update 2018-09
Pension Plan and Other Postretirement Benefits Plan
Pension and other postretirement plan investments impacted by ASU 2018-09		$ 1,200	$ 973