Summary of Significant Accounting Policies - Weighted-Average Useful Lives of Structures and Machinery and Equipment (Detail)	12 Months Ended
	Dec. 31, 2016
Structures [Member] | Global Rolled Products [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	31 years
Structures [Member] | Engineered Products and Solutions [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	29 years
Structures [Member] | Transportation and Construction Solutions [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	27 years
Machinery and Equipment [Member] | Global Rolled Products [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	21 years
Machinery and Equipment [Member] | Engineered Products and Solutions [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	17 years
Machinery and Equipment [Member] | Transportation and Construction Solutions [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	19 years