Segment Information - Disaggregation of Revenue by Major End Market Served (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue, Major Customer [Line Items]
Revenues	$ 3,690	$ 3,609	$ 7,230	$ 7,053
Engineered Products and Solutions
Revenue, Major Customer [Line Items]
Revenues	1,565	1,474	3,067	2,900
Global Rolled Products
Revenue, Major Customer [Line Items]
Revenues	1,577	1,573	3,080	3,054
Transportation and Construction Solutions
Revenue, Major Customer [Line Items]
Revenues	548	562	1,083	1,099
Aerospace
Revenue, Major Customer [Line Items]
Revenues	1,611	1,467	3,163	2,856
Aerospace | Engineered Products and Solutions
Revenue, Major Customer [Line Items]
Revenues	1,283	1,187	2,533	2,328
Aerospace | Global Rolled Products
Revenue, Major Customer [Line Items]
Revenues	328	280	630	528
Aerospace | Transportation and Construction Solutions
Revenue, Major Customer [Line Items]
Revenues	0	0	0	0
Transportation
Revenue, Major Customer [Line Items]
Revenues	975	984	1,966	1,922
Transportation | Engineered Products and Solutions
Revenue, Major Customer [Line Items]
Revenues	85	93	172	166
Transportation | Global Rolled Products
Revenue, Major Customer [Line Items]
Revenues	632	638	1,281	1,260
Transportation | Transportation and Construction Solutions
Revenue, Major Customer [Line Items]
Revenues	258	253	513	496
Building and construction
Revenue, Major Customer [Line Items]
Revenues	345	357	675	690
Building and construction | Engineered Products and Solutions
Revenue, Major Customer [Line Items]
Revenues	0	0	0	0
Building and construction | Global Rolled Products
Revenue, Major Customer [Line Items]
Revenues	54	60	103	108
Building and construction | Transportation and Construction Solutions
Revenue, Major Customer [Line Items]
Revenues	291	297	572	582
Industrial and Other
Revenue, Major Customer [Line Items]
Revenues	759	801	1,426	1,585
Industrial and Other | Engineered Products and Solutions
Revenue, Major Customer [Line Items]
Revenues	197	194	362	406
Industrial and Other | Global Rolled Products
Revenue, Major Customer [Line Items]
Revenues	563	595	1,066	1,158
Industrial and Other | Transportation and Construction Solutions
Revenue, Major Customer [Line Items]
Revenues	$ (1)	$ 12	$ (2)	$ 21