Pension and Other Postretirement Benefits - Schedule of Obligations and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Pension benefits
Change in benefit obligation
Benefit obligation at beginning of year	$ 7,249	$ 6,476
Transfer to Arconic Corporation	(4,355)	0
Service cost	6	25
Interest cost	71	235
Amendments	6	0
Actuarial losses	313	974
Settlements	(398)	(23)
Benefits paid	(153)	(477)
Medicare Part D subsidy receipts	0	0
Foreign currency translation impact	(26)	39
Benefit obligation at end of year	2,713	7,249
Change in plan assets
Fair value of plan assets at beginning of year	4,868	4,334
Transfer to Arconic Corporation	(2,982)	0
Actual return on plan assets	203	731
Employer contributions	227	268
Benefits paid	(136)	(453)
Administrative expenses	(12)	(34)
Settlement payments	(413)	(22)
Foreign currency translation impact	(31)	44
Fair value of plan assets at end of year	1,724	4,868
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status	(989)	(2,381)
Amounts recognized in the Consolidated Balance Sheet consist of:
Noncurrent assets	12	41
Noncurrent assets of discontinued operations	0	63
Current liabilities	(16)	(19)
Current liabilities of discontinued operations	0	(7)
Noncurrent liabilities	(985)	(1,030)
Noncurrent liabilities of discontinued operations	0	(1,429)
Net amount recognized	(989)	(2,381)
Amounts recognized in Accumulated Other Comprehensive Loss consist of:
Net actuarial loss	1,274	3,375
Prior service cost (benefit)	6	1
Net amount recognized, before tax effect	1,280	3,376
Other changes in plan assets and benefit obligations recognized in Other Comprehensive Loss consist of:
Net actuarial loss	166	566
Amortization of accumulated net actuarial (loss) gain	(123)	(148)
Loss transferred to Arconic Corporation	(2,144)	0
Prior service cost (benefit)	5	0
Amortization of prior service (cost) benefit	0	(2)
Prior service credit transferred to Arconic Corporation	0	0
Net amount recognized, before tax effect	(2,096)	416
Other postretirement benefits
Change in benefit obligation
Benefit obligation at beginning of year	786	806
Transfer to Arconic Corporation	(569)	0
Service cost	2	7
Interest cost	7	28
Amendments	(11)	(78)
Actuarial losses	14	100
Settlements	0	0
Benefits paid	(17)	(82)
Medicare Part D subsidy receipts	3	5
Foreign currency translation impact	0	0
Benefit obligation at end of year	215	786
Change in plan assets
Fair value of plan assets at beginning of year	0	0
Transfer to Arconic Corporation	0	0
Actual return on plan assets	0	0
Employer contributions	0	0
Benefits paid	0	0
Administrative expenses	0	0
Settlement payments	0	0
Foreign currency translation impact	0	0
Fair value of plan assets at end of year	0	0
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status	(215)	(786)
Amounts recognized in the Consolidated Balance Sheet consist of:
Noncurrent assets	0	0
Noncurrent assets of discontinued operations	0	0
Current liabilities	(17)	(17)
Current liabilities of discontinued operations	0	(55)
Noncurrent liabilities	(198)	(200)
Noncurrent liabilities of discontinued operations	0	(514)
Net amount recognized	(215)	(786)
Amounts recognized in Accumulated Other Comprehensive Loss consist of:
Net actuarial loss	22	179
Prior service cost (benefit)	(28)	(37)
Net amount recognized, before tax effect	(6)	142
Other changes in plan assets and benefit obligations recognized in Other Comprehensive Loss consist of:
Net actuarial loss	14	100
Amortization of accumulated net actuarial (loss) gain	1	(8)
Loss transferred to Arconic Corporation	(170)	0
Prior service cost (benefit)	(11)	(78)
Amortization of prior service (cost) benefit	5	68
Prior service credit transferred to Arconic Corporation	13	0
Net amount recognized, before tax effect	(148)	82
United States | Pension benefits
Change in benefit obligation
Benefit obligation at beginning of year	5,884
Benefit obligation at end of year	2,327	5,884
Change in plan assets
Fair value of plan assets at beginning of year	3,513
Fair value of plan assets at end of year	1,361	3,513
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status	$ (966)	$ (2,371)