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Asset Retirement Obligations (Rollforward) (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Asset Retirement Obligation Roll Forward Analysis [Roll Forward]    
Beginning asset retirement obligations $ 126,301us-gaap_AssetRetirementObligation $ 106,430us-gaap_AssetRetirementObligation
Liabilities incurred and assumed during period 7,798us-gaap_AssetRetirementObligationLiabilitiesIncurred 22,216us-gaap_AssetRetirementObligationLiabilitiesIncurred
Revisions in estimated retirement obligations (1,298)us-gaap_AssetRetirementObligationRevisionOfEstimate 4,730us-gaap_AssetRetirementObligationRevisionOfEstimate
Liabilities settled and sold during period (13,576)us-gaap_AssetRetirementObligationLiabilitiesSettled (15,523)us-gaap_AssetRetirementObligationLiabilitiesSettled
Accretion expense 8,870us-gaap_AssetRetirementObligationAccretionExpense 8,448us-gaap_AssetRetirementObligationAccretionExpense
Ending asset retirement obligations 128,095us-gaap_AssetRetirementObligation 126,301us-gaap_AssetRetirementObligation
Less: current asset retirement obligations (1,684)us-gaap_AssetRetirementObligationCurrent [1] (6,413)us-gaap_AssetRetirementObligationCurrent [1]
Long-term asset retirement obligations $ 126,411us-gaap_AssetRetirementObligationsNoncurrent $ 119,888us-gaap_AssetRetirementObligationsNoncurrent
[1] Included in "Accounts payable and accrued liabilities" in our Consolidated Balance Sheets.