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, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	$ 281,263	$ 279,864	$ 263,489	$ 263,847
Contractual Third- Party Reclamation Receivable	96,249
Reclamation deposits	75,315	74,921
Rosebud Mine
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	127,989
Contractual Third- Party Reclamation Receivable	21,656
Reclamation deposits	75,315
Jewett Mine
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	74,247
Contractual Third- Party Reclamation Receivable	74,247
Reclamation deposits	0
Absaloka Mine
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	37,286
Contractual Third- Party Reclamation Receivable	346
Reclamation deposits	0
Beulah Mine
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	17,764
Contractual Third- Party Reclamation Receivable	0
Reclamation deposits	0
Kemmerer Mine
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	17,569
Contractual Third- Party Reclamation Receivable	0
Reclamation deposits	0
Savage
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	5,499
Contractual Third- Party Reclamation Receivable	0
Reclamation deposits	0
ROVA
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	909
Contractual Third- Party Reclamation Receivable	0
Reclamation deposits	$ 0