The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

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-Glossary Asset Retirement Obligation

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- Definition

Amount of credit risk valuation adjustment to derivative assets to properly reflect the credit quality of the counterparties.

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Amount of credit risk valuation adjustment to derivative liabilities to properly reflect the credit quality of the entity.

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- Definition

The expense recorded to reduce the value of oil and gas assets consisting of proved properties and unproved properties as the estimate of future successful production from these properties is reduced.

+ References

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

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-SubTopic 10

-Section 45

-Paragraph 28

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- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

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-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 20, 22

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-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

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The fair value amount of long-term debt 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.

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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.

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-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22)

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-Number 210

-Section 02

-Paragraph 22

-Article 5

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- Definition

Fair value portion of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

+ References

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-Topic 820

-SubTopic 10

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-Paragraph 1

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Fair Value Measurements - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Net derivative asset	$ (600,000)
Derivative liability, net		200,000
Long-term debt, carrying value	2,700,000,000us-gaap_LongTermDebtNoncurrent	2,535,570,000us-gaap_LongTermDebtNoncurrent
Asset retirement obligation, carrying value	42,549,000us-gaap_AssetRetirementObligation	36,458,000us-gaap_AssetRetirementObligation	23,234,000us-gaap_AssetRetirementObligation
Impairment of oil and gas properties	47,238,000us-gaap_ImpairmentOfOilAndGasProperties	1,168,000us-gaap_ImpairmentOfOilAndGasProperties	3,581,000us-gaap_ImpairmentOfOilAndGasProperties
Senior unsecured notes [Member]
Debt Instrument [Line Items]
Long-term, unsecured notes and borrowings	2,200,000,000
Long-term debt, fair value	2,020,000,000
Amended Credit Facility [Member]
Debt Instrument [Line Items]
Long-term, unsecured notes and borrowings	500,000,000
Proved oil and gas properties [Member]
Debt Instrument [Line Items]
Impairment of oil and gas properties	40,000,000	0	0
Reported Value Measurement [Member] \| Proved oil and gas properties [Member]
Debt Instrument [Line Items]
Fair value of property	76,400,000
Estimate of Fair Value Measurement [Member] \| Proved oil and gas properties [Member]
Debt Instrument [Line Items]
Fair value of property	$ 36,400,000