Acquisitions and Divestitures Acquisitions and Divestitures - 2019 Divestitures (Details) $ in Millions				3 Months Ended	12 Months Ended
	Feb. 01, 2020 USD ($) employee	Dec. 01, 2019 USD ($) employee	Aug. 15, 2019 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Oct. 30, 2019 USD ($)	May 31, 2019 USD ($)
Business Acquisition [Line Items]
Gain (loss) on disposition of business, before tax					$ 20	$ 109	$ (57)
Non-cash asset impairments					570	13	58
Restricted cash					55	6	4
Restructuring and other charges					620	9	165
Small Additive Manufacturing Facility | Engineered Products and Forgings
Business Acquisition [Line Items]
Gain (loss) on disposition of business, before tax					(13)
Consideration									$ 1
Proceeds from divestiture of businesses			$ 13
Non-cash asset impairments					10
Alloy Extrusions | Global Rolled Products
Business Acquisition [Line Items]
Consideration								$ 61
Forgings Business | Engineered Products and Forgings
Business Acquisition [Line Items]
Gain (loss) on disposition of business, before tax		$ (46)
Consideration		$ 64
Restricted cash					53
Sales					116	131	127
Aluminum Rolling Mill | Global Rolled Products
Business Acquisition [Line Items]
Non-cash asset impairments					53
Sales					$ 143	$ 179	$ 162
Number of employees | employee		540
Scenario, Forecast | Minimum | Alloy Extrusions | Global Rolled Products
Business Acquisition [Line Items]
Gain (loss) on disposition of business, before tax				$ 25
Scenario, Forecast | Maximum | Alloy Extrusions | Global Rolled Products
Business Acquisition [Line Items]
Gain (loss) on disposition of business, before tax				$ 30
Subsequent Event | Aluminum Rolling Mill | Global Rolled Products
Business Acquisition [Line Items]
Consideration	$ 50
Number of employees | employee	513