RESTRICTED INVESTMENTS AND BOND COLLATERAL - Components of Restricted Investments and Bond Collateral (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	$ 540,383	$ 75,233
Less current portion	(16,900)	(5,998)
Total restricted investments and bond collateral, less current portion	523,483	69,235
Debt Reserve Account | Coal Segment
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	13,068	13,067
Reclamation Bond Collateral | Coal Segment | Kemmerer Mine
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	24,944	24,966
Reclamation Bond Collateral | Coal Segment | Absaloka Mine
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	11,670	11,653
Reclamation Bond Collateral | Coal Segment | Rosebud Mine
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	3,145	3,145
Reclamation Bond Collateral | Coal Segment | Beulah Mine
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	1,270	1,270
Letter of Credit Account | Power Segment
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	0	5,998
Proceeds From New Notes Held In Escrow | Corporate Segment
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	454,219	0
Other Requirements for Sherritt Acquisition Held in Escrow | Corporate Segment
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	16,900	0
Postretirement Medical Benefit Bonds | Corporate Segment
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	8,481	8,467
Workers' Compensation Bonds | Corporate Segment
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	$ 6,686	$ 6,667