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Asset Retirement Obligation - Schedule of Asset Retirement Obligation (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Asset Retirement Obligation Disclosure [Abstract]    
Balance at January 1, $ 5,032 $ 8,882
Accretion 38 82
Payments (176) (662)
Gain (43)  
Balance at March 31, $ 4,851 $ 8,302