Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
Sep. 30, 2011	Dec. 31, 2010
Fair value of plan assets
Fair value of money market funds	$ 0.1
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair value of plan assets
Fair value of money market funds	0.1
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Natural Gas Derivatives [Member]
Fair value of plan assets
Fair value of plan assets, Derivatives	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Oil Derivatives [Member]
Fair value of plan assets
Fair value of plan assets, Derivatives	0
Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets
Fair value of money market funds	0
Significant Other Observable Inputs (Level 2) [Member] \| Natural Gas Derivatives [Member]
Fair value of plan assets
Fair value of plan assets, Derivatives	0
Significant Other Observable Inputs (Level 2) [Member] \| Oil Derivatives [Member]
Fair value of plan assets
Fair value of plan assets, Derivatives	0
Significant Unobservable Inputs (Level 3) [Member]
Fair value of plan assets
Fair value of money market funds	0
Significant Unobservable Inputs (Level 3) [Member] \| Natural Gas Derivatives [Member]
Fair value of plan assets
Fair value of plan assets, Derivatives	0
Significant Unobservable Inputs (Level 3) [Member] \| Oil Derivatives [Member]
Fair value of plan assets
Fair value of plan assets, Derivatives	0
Natural Gas Derivatives [Member]
Fair value of plan assets
Fair value of plan assets, Derivatives	0
Oil Derivatives [Member]
Fair value of plan assets
Fair value of plan assets, Derivatives	0
Senior notes due 2017 [Member]
Fair Value Measurements (Textual) [Abstract]
Carrying value of senior notes	250.0	250.0
Percentage of face value amount for measuring senior notes at their fair value	98.00%	101.90%
Debt Instrument, Fair Value Disclosure	245.0	254.7
Senior notes due 2020 [Member]
Fair Value Measurements (Textual) [Abstract]
Carrying value of senior notes	221.8	221.6
Percentage of face value amount for measuring senior notes at their fair value	106.00%	107.70%
Debt Instrument, Fair Value Disclosure	$ 237.4	$ 242.3

Fair Value Measurements.

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Percentage of face value amount for measuring senior notes at their fair value.

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+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Fair value as of the balance sheet date of the assets less the liabilities of a derivative or group of derivatives.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

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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. This item represents investment securities as of the balance sheet date which may include marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

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-Name Statement of Financial Accounting Standard (FAS)

-Number 115

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-Glossary Trading Securities

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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

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