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	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	$ 389,384	$ 279,864	$ 263,151	$ 263,847
Contractual Third- Party Reclamation Receivable	101,896
Reclamation deposits	75,911	74,921
Coal - U.S. Segment
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	281,346
Contractual Third- Party Reclamation Receivable	95,157
Reclamation deposits	75,911
Coal - Canada Segment
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	107,111
Contractual Third- Party Reclamation Receivable	6,739
Reclamation deposits	0
Power Segment
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	927
Contractual Third- Party Reclamation Receivable	0
Reclamation deposits	$ 0