Revenue from Contracts with Customers - Segment Revenue By Product (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	$ 3,522	$ 3,234	$ 10,575	$ 9,685
Aerospace
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	1,464	1,344	4,320	4,096
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	972	802	2,894	2,333
Building and construction
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	352	342	1,042	987
Industrial and Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	734	746	2,319	2,269
Engineered Products and Solutions
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	1,566	1,477	4,703	4,449
Engineered Products and Solutions | Aerospace
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	1,244	1,130	3,675	3,426
Engineered Products and Solutions | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	110	98	326	288
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	212	249	702	735
Global Rolled Products
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	1,426	1,234	4,243	3,753
Global Rolled Products | Aerospace
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	220	214	645	670
Global Rolled Products | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	625	490	1,835	1,458
Global Rolled Products | Building and construction
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	59	56	167	156
Global Rolled Products | Industrial and Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	522	474	1,596	1,469
Transportation and Construction Solutions
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	530	523	1,629	1,483
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	237	214	733	587
Transportation and Construction Solutions | Building and construction
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	293	286	875	831
Transportation and Construction Solutions | Industrial and Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	$ 0	$ 23	$ 21	$ 65