RESTRICTED INVESTMENTS AND BOND COLLATERAL - Components of Restricted Investments and Bond Collateral (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	$ 129,813	$ 164,389
Coal - Canada Segment | CVRI
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	31,869	31,866
Coal - Canada Segment | PMRU
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	18,209	18,199
Reclamation Bond Collateral | Coal - U.S. Segment | Kemmerer Mine
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	25,692	25,282
Reclamation Bond Collateral | Coal - U.S. Segment | Absaloka Mine
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	11,799	11,781
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
Power Contract Collateral | Power Segment
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	12,600	12,600
Postretirement Medical Benefit Bonds | Corporate Segment
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	8,834	8,780
Workers' Compensation Bonds | Corporate Segment
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	6,848	6,832
Buckingham acquisition escrow | Reclamation Bond Collateral | Coal - U.S. Segment
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	0	34,000
Westmoreland Resource Partners LP | Coal - WMLP Segment
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	$ 9,547	$ 10,634