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Components of Changes in Asset Retirement Obligation Liability (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Asset Retirement Obligation [Line Items]    
Asset retirement obligation, January 1 $ 5,095 $ 4,884
Accretion expense 50 211
Asset retirement obligation, December 31 $ 5,145 $ 5,095