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Asset Retirement Obligations - Changes in Asset Retirement Obligation Liabilities (Detail) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2014
Asset Retirement Obligation Liabilities [Line Items]  
Beginning balance $ 7,957
Accretion 564
Additions 141
Reclassifications 0
Amounts paid (342)
Ending balance 8,320
Short-Term Portion [Member]
 
Asset Retirement Obligation Liabilities [Line Items]  
Beginning balance 358
Accretion 21
Additions 0
Reclassifications 373
Amounts paid (342)
Ending balance 410
Long-Term Portion [Member]
 
Asset Retirement Obligation Liabilities [Line Items]  
Beginning balance 7,599
Accretion 543
Additions 141
Reclassifications (373)
Amounts paid 0
Ending balance $ 7,910