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, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 4,448	$ 4,739
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	5,852		$ 5,253
Service cost	228	195	112
Interest cost	217	220	200
Actuarial (gain) loss	(601)	525
Gross benefits paid and settlements	(292)	(366)
Other	(8)	25
Projected benefit obligation at end of year	5,396	5,852
Change in plan assets:
Fair value of plan assets at beginning of year	3,932		3,355
Actual (loss) return on plan assets	(215)	510
Employer contributions	413	419
Gross benefits paid and settlements	(292)	(366)
Other	(11)	14
Fair value of plan assets at end of year	3,827	3,932
Funded status—Net amount recognized	(1,569)	(1,920)
Amounts recognized in the consolidated balance sheets consist of:
Noncurrent asset	13	9
Current liability	(6)	(8)
Noncurrent liability	(1,576)	(1,921)
Total liability	(1,569)	(1,920)
Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	(1,382)	(1,610)
Prior service cost	(5)	(1)
Total accumulated other comprehensive loss	(1,387)	(1,611)
Other Postretirement Benefits
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	1,710		1,687
Service cost	12	13	19
Interest cost	61	66	86
Plan participants' contributions	68	68
Actuarial (gain) loss	(285)	40
Benefits paid	(181)	(178)
Other	6	14
Projected benefit obligation at end of year	1,391	1,710
Change in plan assets:
Fair value of plan assets at beginning of year	54		$ 55
Actual (loss) return on plan assets	1	1
Employer contributions	111	108
Plan participants' contributions	68	68
Benefits paid	(181)	(178)
Fair value of plan assets at end of year	53	54
Funded status—Net amount recognized	(1,338)	(1,656)
Amounts recognized in the consolidated balance sheets consist of:
Current liability	(43)	(54)
Noncurrent liability	(1,295)	(1,602)
Total liability	(1,338)	(1,656)
Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	554	301
Prior service cost	170	208
Total accumulated other comprehensive loss	$ 724	$ 509