RESTRICTED INVESTMENTS AND BOND COLLATERAL - Components of Restricted Investments and Bond Collateral (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	$ 108,734	$ 75,233
Less current portion	0	(5,998)
Total restricted investments and bond collateral, less current portion	108,734	69,235
Coal - Canada Segment | CVRI
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	31,867	0
Coal - Canada Segment | PMRU
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	16,203	0
Debt Reserve Account | Coal - U.S. Segment
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	0	13,067
Reclamation Bond Collateral | Coal - U.S. Segment | Kemmerer Mine
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	25,544	24,966
Reclamation Bond Collateral | Coal - U.S. Segment | Absaloka Mine
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	11,775	11,653
Reclamation Bond Collateral | Coal - U.S. Segment | Rosebud Mine
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	3,145	3,145
Reclamation Bond Collateral | Coal - U.S. 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	3,400	5,998
Postretirement Medical Benefit Bonds | Corporate Segment
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	8,707	8,467
Workers' Compensation Bonds | Corporate Segment
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	$ 6,823	$ 6,667