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, 2013	Dec. 31, 2012	Dec. 31, 2011
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	$ 279,864,000	$ 263,847,000	$ 247,478,000
Contractual Third- Party Reclamation Receivable	96,790,000
Reclamation Deposits	74,921,000	72,718,000
surety bonds outstanding to secure reclamation obligations	298,600,000
Rosebud Mine [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	126,965,000
Contractual Third- Party Reclamation Receivable	21,186,000
Reclamation Deposits	74,921,000
Jewett Mine [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	75,267,000
Contractual Third- Party Reclamation Receivable	75,267,000
Reclamation Deposits	0
Absaloka Mine [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	36,401,000
Contractual Third- Party Reclamation Receivable	337,000
Reclamation Deposits	0
Beulah Mine [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	17,785,000
Contractual Third- Party Reclamation Receivable	0
Reclamation Deposits	0
Kemmerer Mine [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	17,174,000
Contractual Third- Party Reclamation Receivable	0
Reclamation Deposits	0
Savage [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	5,380,000
Contractual Third- Party Reclamation Receivable	0
Reclamation Deposits	0
ROVA [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	892,000
Contractual Third- Party Reclamation Receivable	0
Reclamation Deposits	$ 0