Revenue from Contracts with Customers - Segment Revenue By Product (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	$ 3,444	$ 3,191
Aerospace
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	1,389	1,370
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	938	759
Building and construction
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	333	311
Industrial and other
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	784	751
Engineered Products and Solutions
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	1,541	1,487
Engineered Products and Solutions | Aerospace
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	1,190	1,155
Engineered Products and Solutions | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	97	93
Engineered Products and Solutions | Building and construction
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	0	0
Engineered Products and Solutions | Industrial and other
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	254	239
Global Rolled Products
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	1,366	1,248
Global Rolled Products | Aerospace
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	199	215
Global Rolled Products | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	598	493
Global Rolled Products | Building and construction
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	48	49
Global Rolled Products | Industrial and other
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	521	491
Transportation and Construction Solutions
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	537	456
Transportation and Construction Solutions | Aerospace
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	0	0
Transportation and Construction Solutions | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	243	173
Transportation and Construction Solutions | Building and construction
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	285	262
Transportation and Construction Solutions | Industrial and other
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	$ 9	$ 21