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Asset Retirement Obligation - Schedule of Asset Retirement Obligation (Detail) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Asset Retirement Obligation Disclosure [Abstract]    
Balance at January 1, $ 8,882 $ 9,679
Accretion 234 347
Payments (2,719) (945)
Gain (1,255)   
Balance at September 30, $ 5,142 $ 9,081