Asset Retirement Obligations - Changes in Asset Retirement Obligations (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Asset Retirement Obligation Disclosure [Abstract]
Asset retirement obligations as of the beginning of the year, including current portion	$ 502,513us-gaap_AssetRetirementObligation	$ 488,302us-gaap_AssetRetirementObligation	$ 425,779us-gaap_AssetRetirementObligation
Liabilities incurred	28,606	19,043	3,869
Liabilities settled	(55,839)	(79,695)	(67,641)
Liabilities assumed			15,263
Divestment of properties	(137,801)	(9,245)	(7,563)
Accretion expense	28,411	33,575	33,331
Revision of estimates	(49,481)	50,533	85,264
Asset retirement obligations as of the end of the year, including current portion	$ 316,409us-gaap_AssetRetirementObligation	$ 502,513us-gaap_AssetRetirementObligation	$ 488,302us-gaap_AssetRetirementObligation

Amount of asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

No definition available.

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- Definition

Amount of asset retirement obligations assumed in connection with the acquisition of a long-lived asset.

+ References

No definition available.

+ Details

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- Definition

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.

+ References

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)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Asset Retirement Obligation

-URI http://asc.fasb.org/extlink&oid=6505190

+ Details

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- Definition

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)(3)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

+ Details

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- Definition

Amount of asset retirement obligations incurred during the period.

+ References

+ Details

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- Definition

Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

+ References

+ Details

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- Definition

Amount of the Increase or Decrease in the amount of the asset retirement obligation during the current period for changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

+ References

+ Details

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