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	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	$ 449,607	$ 452,745	$ 384,807	$ 279,864
Contractual Third- Party Reclamation Receivable	115,396
Reclamation deposits	77,425
Coal - U.S. Segment
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	279,864
Contractual Third- Party Reclamation Receivable	109,758
Reclamation deposits	77,425
Coal - Canada Segment
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	116,061
Contractual Third- Party Reclamation Receivable	5,638
Reclamation deposits	0
Coal - WMLP Segment
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	52,666
Contractual Third- Party Reclamation Receivable	0
Reclamation deposits	0
Power Segment
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	1,016
Contractual Third- Party Reclamation Receivable	0
Reclamation deposits	$ 0