RESTRICTED INVESTMENTS AND BOND COLLATERAL - Components of Restricted Investments and Bond Collateral (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	$ 137,672	$ 164,389
Coal - Canada Segment | PMRU
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	18,220	18,199
Coal - Canada Segment | CVRI
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	33,870	31,866
Coal - WMLP Segment | OHIO
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	8,255	10,634
Coal - WMLP Segment | Kemmerer Mine
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	27,655	25,282
Reclamation Bond Collateral | Coal - U.S. Segment | Absaloka Mine
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	11,807	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	17,700	12,600
Postretirement Medical Benefit Bonds | Corporate Segment
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	8,891	8,780
Workers' Compensation Bonds | Corporate Segment
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	6,859	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