Segment Information - Disaggregation of Revenue by Major End Market Served (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 3,209	$ 3,541
Operating Segments
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	3,209	3,540
Engineered Products and Forgings
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,631	1,756
Global Rolled Products
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,578	1,784
Global Rolled Products | Operating Segments
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,578	1,784
Aerospace | Operating Segments
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,474	1,552
Aerospace | Engineered Products and Forgings
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,189	1,250
Aerospace | Global Rolled Products
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	285	302
Transportation | Operating Segments
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	765	992
Transportation | Engineered Products and Forgings
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	256	343
Transportation | Global Rolled Products
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	509	649
Building and construction | Operating Segments
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	291	330
Building and construction | Engineered Products and Forgings
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0	0
Building and construction | Global Rolled Products
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	291	330
Industrial and Other | Operating Segments
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	679	666
Industrial and Other | Engineered Products and Forgings
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	186	163
Industrial and Other | Global Rolled Products
Revenue, Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 493	$ 503