Summary of Significant Accounting Policies - Weighted-Average Useful Lives of Software and Other Intangible Assets (Details)	12 Months Ended
	Dec. 31, 2019
Software | Engineered Products and Forgings
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	5 years
Software | Global Rolled Products
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	5 years
Other intangible assets | Engineered Products and Forgings
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	32 years
Other intangible assets | Global Rolled Products
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	13 years