Revenue from Contracts with Customers - Segment Revenue By Product (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	$ 3,609	$ 3,260	$ 7,053	$ 6,451
Aerospace
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	1,467	1,382	2,856	2,752
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	984	772	1,922	1,531
Building and construction
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	357	334	690	645
Industrial and other
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	801	772	1,585	1,523
Engineered Products and Solutions
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	1,596	1,485	3,137	2,972
Engineered Products and Solutions | Aerospace
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	1,241	1,141	2,431	2,296
Engineered Products and Solutions | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	119	97	216	190
Engineered Products and Solutions | Building and construction
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	0	0	0	0
Engineered Products and Solutions | Industrial and other
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	236	247	490	486
Global Rolled Products
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	1,451	1,271	2,817	2,519
Global Rolled Products | Aerospace
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	226	241	425	456
Global Rolled Products | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	612	475	1,210	968
Global Rolled Products | Building and construction
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	60	51	108	100
Global Rolled Products | Industrial and other
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	553	504	1,074	995
Transportation and Construction Solutions
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	562	504	1,099	960
Transportation and Construction Solutions | Aerospace
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	0	0	0	0
Transportation and Construction Solutions | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	253	200	496	373
Transportation and Construction Solutions | Building and construction
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	297	283	582	545
Transportation and Construction Solutions | Industrial and other
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	$ 12	$ 21	$ 21	$ 42