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Asset Retirement Obligations - Schedule of Changes in Asset Retirement Obligations (Detail) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Asset Retirement Obligation Disclosure [Abstract] |  |
Current portion of total ARO balance | $ 500,000 us-gaap_AssetRetirementObligationCurrent | $ 500,000 us-gaap_AssetRetirementObligationCurrent
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |
Asset retirement obligation - beginning of period | 36,458,000 us-gaap_AssetRetirementObligation | 23,234,000 us-gaap_AssetRetirementObligation
Liabilities incurred during period | 5,376,000 us-gaap_AssetRetirementObligationLiabilitiesIncurred | 11,665,000 us-gaap_AssetRetirementObligationLiabilitiesIncurred
Liabilities settled during period | (2,065,000)us-gaap_AssetRetirementObligationLiabilitiesSettled | 0 us-gaap_AssetRetirementObligationLiabilitiesSettled
Accretion expense during period | 1,917,000 us-gaap_AssetRetirementObligationAccretionExpense | 1,346,000 us-gaap_AssetRetirementObligationAccretionExpense
Revisions to estimates | 863,000 us-gaap_AssetRetirementObligationRevisionOfEstimate | 213,000 us-gaap_AssetRetirementObligationRevisionOfEstimate
Asset retirement obligation - end of period | $ 42,549,000 us-gaap_AssetRetirementObligation | $ 36,458,000 us-gaap_AssetRetirementObligation

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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 Current portion of 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 + Details
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- Definition Amount of asset retirement obligations incurred during the period. + 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)(1) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + 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 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)(2) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + 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 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)(4) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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