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Summary of Significant Accounting Policies - Weighted-Average Useful Lives of Software and Other Intangible Assets (Detail)	12 Months Ended
	Dec. 31, 2016
Software [Member] | Global Rolled Products [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	8 years
Software [Member] | Engineered Products and Solutions [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	7 years
Software [Member] | Transportation and Construction Solutions [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	6 years
Other Intangible Assets [Member] | Global Rolled Products [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	9 years
Other Intangible Assets [Member] | Engineered Products and Solutions [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	34 years
Other Intangible Assets [Member] | Transportation and Construction Solutions [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	16 years

X

- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Details

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X

- Details

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X

- Details

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X

- Details

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- Details

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