Summary of Significant Accounting Policies - Weighted-Average Useful Lives of Structures and Machinery and Equipment (Detail)	12 Months Ended
	Dec. 31, 2017
Structures | Engineered Products and Solutions
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	29 years
Structures | Global Rolled Products
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	31 years
Structures | Transportation and Construction Solutions
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	27 years
Machinery and equipment | Engineered Products and Solutions
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	17 years
Machinery and equipment | Global Rolled Products
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	21 years
Machinery and equipment | Transportation and Construction Solutions
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	18 years