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	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	$ 384,807	$ 279,864	$ 263,201	$ 263,847
Contractual Third- Party Reclamation Receivable	97,914
Reclamation deposits	77,131	74,921
Coal - U.S. Segment
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	279,859
Contractual Third- Party Reclamation Receivable	91,370
Reclamation deposits	77,131
Coal - Canada Segment
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	104,004
Contractual Third- Party Reclamation Receivable	6,544
Reclamation deposits	0
Power Segment
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	944
Contractual Third- Party Reclamation Receivable	0
Reclamation deposits	$ 0