Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010
Senior notes due 2017 [Member]
Fair Value Measurements (Textual) [Abstract]
Carrying value of senior notes	$ 250.0	$ 250.0
Fair value of senior notes	254.8	254.7
Percentage of face value amount for measuring senior notes at their fair value	101.90%	101.90%
Senior notes due 2020 [Member]
Fair Value Measurements (Textual) [Abstract]
Carrying value of senior notes	221.9	221.6
Fair value of senior notes	239.6	242.3
Percentage of face value amount for measuring senior notes at their fair value	106.50%	107.70%
Senior notes due 2022 [Member]
Fair Value Measurements (Textual) [Abstract]
Carrying value of senior notes	247.9
Fair value of senior notes	252.8
Percentage of face value amount for measuring senior notes at their fair value	101.10%
Oil Derivatives [Member]
Fair value of plan assets
Fair value of plan assets, Derivatives	0.1
Natural Gas Derivatives [Member]
Fair value of plan assets
Fair value of plan assets, Derivatives		0.3
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
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
Significant Other Observable Inputs (Level 2) [Member] \| Oil Derivatives [Member]
Fair value of plan assets
Fair value of plan assets, Derivatives	0.1
Significant Other Observable Inputs (Level 2) [Member] \| Natural Gas Derivatives [Member]
Fair value of plan assets
Fair value of plan assets, Derivatives		0.3
Significant Unobservable Inputs (Level 3) [Member] \| Oil Derivatives [Member]
Fair value of plan assets
Fair value of plan assets, Derivatives	0
Significant Unobservable Inputs (Level 3) [Member] \| Natural Gas Derivatives [Member]
Fair value of plan assets
Fair value of plan assets, Derivatives		$ 0

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

+ 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.

+ 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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- 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

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 SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

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-Subparagraph (SX 210.5-02.22)

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