Segment Information - Assets By Reportable Segment (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total segment assets	$ 18,327	$ 18,718
Operating Segments
Segment Reporting Information [Line Items]
Total segment assets	15,725	15,321
Engineered Products and Solutions
Segment Reporting Information [Line Items]
Total segment assets	10,522	10,325
Global Rolled Products
Segment Reporting Information [Line Items]
Total segment assets	4,112	3,955
Transportation and Construction Solutions
Segment Reporting Information [Line Items]
Total segment assets	$ 1,091	$ 1,041