Fair Value Of Financial Instruments (Carrying Value And Fair Value Of Financial Instruments Disclosure) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Long-term debt:
Long-term debt noncurrent fair value	$ 3,614.1	$ 3,608.7
Fair Value [Member]
Receivables:
Receivables	46.8	61.5
Long-term debt:
Revolving loan	325.0
Long-term debt noncurrent fair value	3,976.1	3,820.5
Fair Value [Member] \| Customer Supplemental Payments [Member]
Receivables:
Receivables	10.5	20.8
Fair Value [Member] \| ArcelorMittal USA - Receivable [Member]
Receivables:
Receivables	26.2	30.7
Fair Value [Member] \| Other Credit Receivable [Member]
Receivables:
Receivables	10.1	10.0
Carrying Value [Member]
Receivables:
Receivables	44.2	58.8
Long-term debt:
Revolving loan	325.0
Long-term debt noncurrent fair value	3,614.1	3,608.7
Carrying Value [Member] \| Customer Supplemental Payments [Member]
Receivables:
Receivables	11.1	22.3
Carrying Value [Member] \| ArcelorMittal USA - Receivable [Member]
Receivables:
Receivables	23.0	26.5
Carrying Value [Member] \| Other Credit Receivable [Member]
Receivables:
Receivables	10.1	10.0
Term Loan - $1.25 Billion [Member] \| Fair Value [Member]
Long-term debt:
Term loan	847.3	897.2
Term Loan - $1.25 Billion [Member] \| Carrying Value [Member]
Long-term debt:
Term loan	847.3	897.2
Senior Notes - $700 Million [Member] \| Fair Value [Member]
Long-term debt:
Senior notes	753.8	726.4
Senior Notes - $700 Million [Member] \| Carrying Value [Member]
Long-term debt:
Senior notes	699.4	699.3
Senior Notes - $1.3 Billion [Member] \| Fair Value [Member]
Long-term debt:
Senior notes	1,526.4	1,399.4
Senior Notes - $1.3 Billion [Member] \| Carrying Value [Member]
Long-term debt:
Senior notes	1,289.3	1,289.2
Senior Notes - $400 Million [Member] \| Fair Value [Member]
Long-term debt:
Senior notes	461.5	448.8
Senior Notes - $400 Million [Member] \| Carrying Value [Member]
Long-term debt:
Senior notes	398.1	398.0
Senior Notes - $55 Million [Member] \| Fair Value [Member]
Long-term debt:
Senior notes	62.1	348.7
Senior Notes - $55 Million [Member] \| Carrying Value [Member]
Long-term debt:
Senior notes	$ 55.0	$ 325.0

Line Of Credit Facility Revolving Loan

No definition available.

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- Definition

Line Of Credit Facility Revolving Loan Fair Value

+ References

No definition available.

+ Details

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- Definition

Fair value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion. Senior note holders are paid off in full before any payments are made to junior note holders.

+ References

No definition available.

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- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. Amounts due from customers or clients for goods or services (including trade receivables) that have been delivered or sold in the normal course of business, as well as any other amounts due from others, including related parties, which may be included in accounts receivable as categorized on the statement of financial position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 14

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.3)

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 3

-Subparagraph a, b

-Article 5

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- Definition

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No definition available.

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- Definition

The aggregate of amounts due from customers or clients more than one year from the balance sheet date, for goods or services that have been delivered or sold in the normal course of business and an amount representing an agreement for an unconditional promise by the maker to pay the Company (holder) a definite sum of money at a future date more than one year from the balance sheet date, reduced to their estimated net realizable fair value by an allowance established by the Company of the amount it deems uncertain of collection and net of any write-downs taken for collection uncertainty on the part of the holder, respectively.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.17)

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 17

-Article 5

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- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22)

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-Publisher SEC

-Name Regulation S-X (SX)

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-Paragraph 22

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- Definition

Carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion. Senior note holders are paid off in full before any payments are made to junior note holders.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22)

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-Publisher SEC

-Name Regulation S-X (SX)

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