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 $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	$ 263,489	$ 263,847	$ 258,696	$ 247,478
Contractual Third- Party Reclamation Receivable	94,629
Reclamation deposits	73,100	72,718
Rosebud Mine
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	118,441
Contractual Third- Party Reclamation Receivable	17,808
Reclamation deposits	73,100
Jewett Mine [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	76,426
Contractual Third- Party Reclamation Receivable	76,426
Reclamation deposits	0
Absaloka Mine
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	26,439
Contractual Third- Party Reclamation Receivable	395
Reclamation deposits	0
Beulah Mine
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	18,441
Contractual Third- Party Reclamation Receivable	0
Reclamation deposits	0
Kemmerer Mine
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	18,006
Contractual Third- Party Reclamation Receivable	0
Reclamation deposits	0
Savage [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	4,891
Contractual Third- Party Reclamation Receivable	0
Reclamation deposits	0
ROVA [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	845
Contractual Third- Party Reclamation Receivable	0
Reclamation deposits	$ 0