Pension and Other Postretirement Plans - Reconciliation of the Change in Benefit Obligation and Plan Asset, Funded Status and Amounts Recognized in the Financial Statements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 5,496	$ 5,387
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	6,525	6,398
Service cost	239	216	$ 184
Interest cost	184	209	226
Actuarial (gain) loss	(188)	1,181
Special termination benefit	0	54
Benefits paid	(263)	(1,445)
Curtailment	(12)	(105)
Other	(12)	17
Projected benefit obligation at end of year	6,473	6,525	6,398
Change in plan assets:
Fair value of plan assets at beginning of year	4,069	4,964
Actual return on plan assets	437	521
Employer contributions	387	16
Benefits paid	(263)	(1,445)
Other	(4)	13
Fair value of plan assets at end of year	4,626	4,069	4,964
Funded status—Net amount recognized	(1,847)	(2,456)
Amounts recognized in the consolidated balance sheets consist of:
Noncurrent asset	75	8
Current liability	(2)	(4)
Noncurrent liability	(1,920)	(2,460)
Total liability	(1,847)	(2,456)
Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	(1,406)	(1,924)
Prior service cost	(1)	(3)
Total accumulated other comprehensive loss	(1,407)	(1,927)
Other Postretirement Benefits
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	1,082	842
Service cost	10	10	10
Interest cost	25	28	47
Actuarial (gain) loss	114	107
Special termination benefit	31	201
Benefits paid	(199)	(164)
Projected benefit obligation at end of year	1,129	1,082	842
Change in plan assets:
Fair value of plan assets at beginning of year	51	52
Actual return on plan assets	1	1
Employer contributions	130	104
Plan participants' contributions	66	58
Benefits paid	(199)	(164)
Fair value of plan assets at end of year	49	51	$ 52
Funded status—Net amount recognized	(1,080)	(1,031)
Amounts recognized in the consolidated balance sheets consist of:
Current liability	(80)	(37)
Noncurrent liability	(1,000)	(994)
Total liability	(1,080)	(1,031)
Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	113	255
Prior service cost	447	570
Total accumulated other comprehensive loss	560	825
Plan participants' contributions	$ 66	$ 58