ASSET RETIREMENT OBLIGATIONS, CONTRACTUAL THIRD-PARTY RECLAMATION RECEIVABLES, AND RECLAMATION DEPOSITS - Summary of Asset Retirement Obligations, Contractual Third-Party Reclamation Receivables, and Reclamation Deposits (Detail) (USD $)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	$ 263,847,000	$ 247,478,000	$ 241,643,000
Contractual Third- Party Reclamation Receivable	94,365,000
Reclamation Deposits	72,718,000	71,939,000
surety bonds outstanding to secure reclamation obligations	316,500,000
Rosebud Mine [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	117,481,000
Contractual Third- Party Reclamation Receivable	17,481,000
Reclamation Deposits	72,718,000
Jewett Mine [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	76,501,000
Contractual Third- Party Reclamation Receivable	76,501,000
Reclamation Deposits	0
Absaloka Mine [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	25,813,000
Contractual Third- Party Reclamation Receivable	383,000
Reclamation Deposits	0
Beulah Mine [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	20,669,000
Contractual Third- Party Reclamation Receivable	0
Reclamation Deposits	0
Kemmerer Mine [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	17,768,000
Contractual Third- Party Reclamation Receivable	0
Reclamation Deposits	0
Savage [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	4,786,000
Contractual Third- Party Reclamation Receivable	0
Reclamation Deposits	0
ROVA [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	829,000
Contractual Third- Party Reclamation Receivable	0
Reclamation Deposits	$ 0