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ASSET RETIREMENT OBLIGATIONS, CONTRACTUAL THIRD-PARTY RECLAMATION RECEIVABLES, AND RECLAMATION DEPOSITS - Changes in Company's Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Dec. 31, 2012
Asset Retirement Obligation Disclosure [Abstract]      
Asset retirement obligations, beginning of period (including current portion) $ 263,847 $ 247,478  
Accretion 16,426 16,360  
Liabilities settled (17,072) (13,814)  
Changes due to amount and timing of reclamation 0 (4,757)  
ARO acquired 0 19,377  
Asset retirement obligations, end of period 263,201 264,644  
Less current portion (25,038) (18,469) (22,238)
Asset retirement obligations, less current portion $ 238,163 $ 246,175 $ 241,609