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Asset Retirement Obligations - Information Related to Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Asset Retirement Obligation Disclosure [Abstract]    
Beginning Balance $ 200,831us-gaap_AssetRetirementObligationsNoncurrent $ 189,659us-gaap_AssetRetirementObligationsNoncurrent
Additions to asset retirement obligations 1,238us-gaap_AssetRetirementObligationLiabilitiesIncurred 3,741us-gaap_AssetRetirementObligationLiabilitiesIncurred
Accretion expense 5,262us-gaap_AssetRetirementObligationAccretionExpense 11,046us-gaap_AssetRetirementObligationAccretionExpense
Liabilities settled (3,004)us-gaap_AssetRetirementObligationLiabilitiesSettled (3,615)us-gaap_AssetRetirementObligationLiabilitiesSettled
Ending Balance $ 204,327us-gaap_AssetRetirementObligationsNoncurrent $ 200,831us-gaap_AssetRetirementObligationsNoncurrent