Acquisitions and Divestitures - 2019 Divestitures (Details) $ in Millions				3 Months Ended	6 Months Ended	12 Months Ended
	Feb. 01, 2020 USD ($)	Dec. 01, 2019 USD ($) employee	Aug. 15, 2019 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	May 31, 2019 USD ($)
Business Acquisition [Line Items]
Gain (loss) on disposition of business, before tax						$ (8)	$ (63)	$ (43)
Non-cash asset impairments (O)						5	442	9
Restructuring and other charges						182	582	163
Restricted cash					$ 55	1	55	6
Small Additive Manufacturing Facility | Engineered Structures
Business Acquisition [Line Items]
Consideration									$ 1
Gain (loss) on disposition of business, before tax							(13)
Proceeds from the sales of businesses			$ 13
Non-cash asset impairments (O)							10
Forgings Business | Engineered Structures
Business Acquisition [Line Items]
Consideration		$ 64
Gain (loss) on disposition of business, before tax							(46)
Restructuring and other charges						5
Restricted cash					53		53
Sales							$ 116	$ 126
Aluminum Rolling Mill
Business Acquisition [Line Items]
Consideration	$ 50
Gain (loss) on disposition of business, before tax	$ (59)
Non-cash asset impairments (O)				$ 6	$ 53	$ 6
Aluminum Rolling Mill | Engine Products
Business Acquisition [Line Items]
Number of employees | employee		540