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Asset Retirement Obligations - Changes in Asset Retirement Obligation Liabilities (Detail) (USD $)
In Thousands, unless otherwise specified
6 Months Ended
Jun. 30, 2014
Asset Retirement Obligation Liabilities [Line Items]  
Beginning balance $ 7,957
Accretion 375
Additions 105
Reclassifications 0
Amounts paid (182)
Ending balance 8,255
Short-Term Portion [Member]
 
Asset Retirement Obligation Liabilities [Line Items]  
Beginning balance 358
Accretion 14
Additions 0
Reclassifications 238
Amounts paid (182)
Ending balance 428
Long-Term Portion [Member]
 
Asset Retirement Obligation Liabilities [Line Items]  
Beginning balance 7,599
Accretion 361
Additions 105
Reclassifications (238)
Amounts paid 0
Ending balance $ 7,827