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, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in projected benefit obligation:
Plan amendments	$ (597)	$ 0
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	5,333	4,448
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	5,396	5,852
Service cost	184	228	$ 195
Interest cost	226	217	220
Actuarial (gain) loss	784	(601)
Gross benefits paid and settlements	(200)	(292)
Other	8	(8)
Projected benefit obligation at end of year	6,398	5,396	5,852
Change in plan assets:
Fair value of plan assets at beginning of year	3,827	3,932
Actual (loss) return on plan assets	684	(215)
Employer contributions	649	413
Gross benefits paid and settlements	(200)	(292)
Other	4	(11)
Fair value of plan assets at end of year	4,964	3,827	3,932
Funded status—Net amount recognized	(1,434)	(1,569)
Amounts recognized in the consolidated balance sheets consist of:
Noncurrent asset	14	13
Current liability	(2)	(6)
Noncurrent liability	(1,446)	(1,576)
Total liability	(1,434)	(1,569)
Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	(1,652)	(1,382)
Prior service cost	(4)	(5)
Total accumulated other comprehensive loss	(1,656)	(1,387)
Other Postretirement Benefits
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	1,391	1,710
Service cost	10	12	13
Interest cost	47	61	66
Plan participants' contributions	67	68
Benefits paid	(180)	(181)
Actuarial (gain) loss	99	(285)
Other	5	6
Projected benefit obligation at end of year	842	1,391	1,710
Change in plan assets:
Fair value of plan assets at beginning of year	53	54
Actual (loss) return on plan assets	1	1
Employer contributions	111	111
Plan participants' contributions	67	68
Benefits paid	(180)	(181)
Fair value of plan assets at end of year	52	53	$ 54
Funded status—Net amount recognized	(790)	(1,338)
Amounts recognized in the consolidated balance sheets consist of:
Current liability	(1)	(43)
Noncurrent liability	(789)	(1,295)
Total liability	(790)	(1,338)
Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	403	554
Prior service cost	693	170
Total accumulated other comprehensive loss	$ 1,096	$ 724