This element represents the fair value of our debt instrument.

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Data Type:	xbrli:stringItemType
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Period Type:	duration

- Definition

Fair Value Measurements.

+ Details

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- Definition

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

+ Details

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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
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13495-108611

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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-Publisher FASB
-Name Accounting Standards Codification
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-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13433-108611

+ Details

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Balance Type:	debit
Period Type:	instant

- 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
-URI http://asc.fasb.org/extlink&oid=6505594

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6379932&loc=d3e22054-111558

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 13
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Trading Securities
-URI http://asc.fasb.org/extlink&oid=6526789

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 12
-Subparagraph a
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 7
-Section 03
-Paragraph 1
-Subparagraph a, f, g

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6956420&loc=d3e24584-111560

+ Details

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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

Reference 2: 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)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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Fair Value Measurements (Details) (USD $) In Millions	6 Months Ended	12 Months Ended
Fair Value Measurements (Details) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Fair value of plan assets
Fair value of money market funds	$ 26.0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair value of plan assets
Fair value of money market funds	26.0
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
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	0.2
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	0.2
Oil Derivatives [Member]
Fair value of plan assets
Fair value of plan assets, Derivatives	0
Senior notes due 2017 [Member]
Fair Value Measurements (Textuals) [Abstract]
Fair value of senior notes	$248.9 million, or 99.6% of face value	254.7 million, or 101.9% of face value
Carrying value of senior notes	250.0	250.0
Senior notes due 2020 [Member]
Fair Value Measurements (Textuals) [Abstract]
Fair value of senior notes	247.5 million, or 110.0% of face value	242.3 million, or 107.7% of face value
Carrying value of senior notes	$ 221.7	$ 221.6