Segment Information - Disaggregation of Revenue by Major End Market Served (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenue, Major Customer [Line Items]
Revenues	$ 3,557	$ 3,522	$ 10,787	$ 10,551
Engineered Products and Forgings
Revenue, Major Customer [Line Items]
Revenues	1,794	1,683	5,372	5,083
Global Rolled Products
Revenue, Major Customer [Line Items]
Revenues	1,763	1,839	5,415	5,468
Aerospace
Revenue, Major Customer [Line Items]
Revenues	1,590	1,464	4,754	4,321
Aerospace | Engineered Products and Forgings
Revenue, Major Customer [Line Items]
Revenues	1,290	1,190	3,823	3,519
Aerospace | Global Rolled Products
Revenue, Major Customer [Line Items]
Revenues	300	274	931	802
Transportation
Revenue, Major Customer [Line Items]
Revenues	906	973	2,872	2,895
Transportation | Engineered Products and Forgings
Revenue, Major Customer [Line Items]
Revenues	319	319	1,004	981
Transportation | Global Rolled Products
Revenue, Major Customer [Line Items]
Revenues	587	654	1,868	1,914
Building and construction
Revenue, Major Customer [Line Items]
Revenues	329	352	1,004	1,033
Building and construction | Engineered Products and Forgings
Revenue, Major Customer [Line Items]
Revenues	0	0	0	0
Building and construction | Global Rolled Products
Revenue, Major Customer [Line Items]
Revenues	329	352	1,004	1,033
Industrial and Other
Revenue, Major Customer [Line Items]
Revenues	732	733	2,157	2,302
Industrial and Other | Engineered Products and Forgings
Revenue, Major Customer [Line Items]
Revenues	185	174	545	583
Industrial and Other | Global Rolled Products
Revenue, Major Customer [Line Items]
Revenues	$ 547	$ 559	$ 1,612	$ 1,719