Restructuring and Other Charges - Activity and Reserve Balances for Restructuring Charges (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	$ 34	$ 25	$ 25
Cash payments	(7)		(79)
Restructuring charges	21		630
Other	(2)		(542)
Restructuring reserve ending balance	46		34
Gain on sale of assets	(2)	(2)
Restructuring charges	21	12
Texarkana, Texas Rolling Mill And Cast House
Restructuring Reserve [Roll Forward]
Gain on sale of assets			20
Extrusions Busines
Restructuring Reserve [Roll Forward]
Gain (loss) on sale of business	27
Layoff costs
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	31	10	10
Cash payments	(7)		(74)
Restructuring charges	20		56
Other	0		39
Restructuring reserve ending balance	44		31
Restructuring charges	22	65
Other exit costs
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	3	15	15
Cash payments	0		(5)
Restructuring charges	1		574
Other	(2)		(581)
Restructuring reserve ending balance	2		3
Restructuring charges	3
Elimination Of Life Insurance Benefits
Restructuring Reserve [Roll Forward]
Other			58
Restructuring charges		(58)
Other, Pension Settlement Cost
Restructuring Reserve [Roll Forward]
Restructuring charges			9
Employee Severance, Reversal Of Prior Restructuring
Restructuring Reserve [Roll Forward]
Other			10
Impairment Of Disks
Restructuring Reserve [Roll Forward]
Restructuring charges			428
Asset Impairment associated with the sale of business
Restructuring Reserve [Roll Forward]
Restructuring charges			112
Other Exit Costs, Impairment Of Intangibles And Property Plant and Equipment
Restructuring Reserve [Roll Forward]
Restructuring charges			25
Other Exit Cost Lease Termination Loss
Restructuring Reserve [Roll Forward]
Restructuring charges			12
Accelerated Depreciation
Restructuring Reserve [Roll Forward]
Restructuring charges			9
Impairment Of A Cost Investment
Restructuring Reserve [Roll Forward]
Restructuring charges			5
Other Miscellaneous Charges
Restructuring Reserve [Roll Forward]
Restructuring charges			1
Restructuring charges		$ 1
Other Exit Costs, Reclassification Of Lease Exit Costs
Restructuring Reserve [Roll Forward]
Restructuring charges			$ 9
Sale Of Assets
Restructuring Reserve [Roll Forward]
Gain on sale of assets	2
Impairments Of Assets Associated With Agreement To Sell Aerospace Components Business
Restructuring Reserve [Roll Forward]
Restructuring charges	12
UK Forgings
Restructuring Reserve [Roll Forward]
Restructuring charges	6
Sale Of Company's Brazilian Rolling Mill Operations
Restructuring Reserve [Roll Forward]
Restructuring charges	$ 6