Revenue from Contracts with Customers (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Major Customer [Line Items]
Costs related to settlements of certain customer claims									$ 38
Sales (B and C)	$ 3,472	$ 3,524	$ 3,573	$ 3,445	$ 3,271	$ 3,236	$ 3,261	$ 3,192	14,014	$ 12,960	$ 12,394
Engineered Products and Solutions
Revenue, Major Customer [Line Items]
Sales (B and C)									6,316	5,943	5,728
Engineered Products and Solutions | Aerospace
Revenue, Major Customer [Line Items]
Sales (B and C)									4,947	4,572	4,358
Engineered Products and Solutions | Transportation
Revenue, Major Customer [Line Items]
Sales (B and C)									440	383	347
Engineered Products and Solutions | Building and construction
Revenue, Major Customer [Line Items]
Sales (B and C)									0	0	0
Engineered Products and Solutions | Industrial and Other
Revenue, Major Customer [Line Items]
Sales (B and C)									929	988	1,023
Global Rolled Products
Revenue, Major Customer [Line Items]
Sales (B and C)									5,604	5,000	4,865
Global Rolled Products | Aerospace
Revenue, Major Customer [Line Items]
Sales (B and C)									891	883	942
Global Rolled Products | Transportation
Revenue, Major Customer [Line Items]
Sales (B and C)									2,443	1,981	1,577
Global Rolled Products | Building and construction
Revenue, Major Customer [Line Items]
Sales (B and C)									217	204	200
Global Rolled Products | Industrial and Other
Revenue, Major Customer [Line Items]
Sales (B and C)									2,053	1,932	2,146
Transportation and Construction Solutions
Revenue, Major Customer [Line Items]
Sales (B and C)									2,126	2,011	1,802
Transportation and Construction Solutions | Aerospace
Revenue, Major Customer [Line Items]
Sales (B and C)									0	0	0
Transportation and Construction Solutions | Transportation
Revenue, Major Customer [Line Items]
Sales (B and C)									969	805	708
Transportation and Construction Solutions | Building and construction
Revenue, Major Customer [Line Items]
Sales (B and C)									1,150	1,113	1,060
Transportation and Construction Solutions | Industrial and Other
Revenue, Major Customer [Line Items]
Sales (B and C)									7	93	34
Total Segment
Revenue, Major Customer [Line Items]
Sales (B and C)									14,206	13,102	12,513
Total Segment | Aerospace
Revenue, Major Customer [Line Items]
Sales (B and C)									5,838	5,455	5,300
Total Segment | Transportation
Revenue, Major Customer [Line Items]
Sales (B and C)									3,852	3,169	2,632
Total Segment | Building and construction
Revenue, Major Customer [Line Items]
Sales (B and C)									1,367	1,317	1,260
Total Segment | Industrial and Other
Revenue, Major Customer [Line Items]
Sales (B and C)									2,989	3,013	3,203
Total Segment | Engineered Products and Solutions
Revenue, Major Customer [Line Items]
Sales (B and C)									6,316	5,943	5,728
Total Segment | Global Rolled Products
Revenue, Major Customer [Line Items]
Sales (B and C)									5,764	5,148	4,983
Total Segment | Transportation and Construction Solutions
Revenue, Major Customer [Line Items]
Sales (B and C)									2,126	2,011	1,802
Third-Party Sales | Total Segment
Revenue, Major Customer [Line Items]
Sales (B and C)									14,046	12,954	12,395
Third-Party Sales | Total Segment | Engineered Products and Solutions
Revenue, Major Customer [Line Items]
Sales (B and C)									6,316	5,943	5,728
Third-Party Sales | Total Segment | Global Rolled Products
Revenue, Major Customer [Line Items]
Sales (B and C)									5,604	5,000	4,865
Third-Party Sales | Total Segment | Transportation and Construction Solutions
Revenue, Major Customer [Line Items]
Sales (B and C)									$ 2,126	$ 2,011	$ 1,802