Segment Information - Disaggregation of Revenue by Major End Market Served (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenue, Major Customer [Line Items]
Revenues	$ 3,540	$ 3,444
Engineered Products and Solutions
Revenue, Major Customer [Line Items]
Revenues	1,502	1,426
Global Rolled Products
Revenue, Major Customer [Line Items]
Revenues	1,503	1,481
Transportation and Construction Solutions
Revenue, Major Customer [Line Items]
Revenues	535	537
Aerospace
Revenue, Major Customer [Line Items]
Revenues	1,552	1,389
Aerospace | Engineered Products and Solutions
Revenue, Major Customer [Line Items]
Revenues	1,250	1,141
Aerospace | Global Rolled Products
Revenue, Major Customer [Line Items]
Revenues	302	248
Aerospace | Transportation and Construction Solutions
Revenue, Major Customer [Line Items]
Revenues	0	0
Transportation
Revenue, Major Customer [Line Items]
Revenues	991	938
Transportation | Engineered Products and Solutions
Revenue, Major Customer [Line Items]
Revenues	87	73
Transportation | Global Rolled Products
Revenue, Major Customer [Line Items]
Revenues	649	622
Transportation | Transportation and Construction Solutions
Revenue, Major Customer [Line Items]
Revenues	255	243
Building and construction
Revenue, Major Customer [Line Items]
Revenues	330	333
Building and construction | Engineered Products and Solutions
Revenue, Major Customer [Line Items]
Revenues	0	0
Building and construction | Global Rolled Products
Revenue, Major Customer [Line Items]
Revenues	49	48
Building and construction | Transportation and Construction Solutions
Revenue, Major Customer [Line Items]
Revenues	281	285
Industrial and Other
Revenue, Major Customer [Line Items]
Revenues	667	784
Industrial and Other | Engineered Products and Solutions
Revenue, Major Customer [Line Items]
Revenues	165	212
Industrial and Other | Global Rolled Products
Revenue, Major Customer [Line Items]
Revenues	503	563
Industrial and Other | Transportation and Construction Solutions
Revenue, Major Customer [Line Items]
Revenues	$ (1)	$ 9