Summary of Significant Accounting Policies - Weighted-Average Useful Lives of Software and Other Intangible Assets (Detail)	12 Months Ended
	Dec. 31, 2017
Software | Engineered Products and Solutions
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	6 years
Software | Global Rolled Products
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	6 years
Software | Transportation and Construction Solutions
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	5 years
Other intangible assets | Engineered Products and Solutions
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	34 years
Other intangible assets | Global Rolled Products
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	9 years
Other intangible assets | Transportation and Construction Solutions
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	16 years