RESTRICTED INVESTMENTS AND BOND COLLATERAL - Components of Restricted Investments and Bond Collateral (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Restricted Investments and Bond Collateral [Line Items]
Restricted investments and bond collateral	$ 87,636	$ 87,209
Debt Reserve Account | Coal Segment
Restricted Investments and Bond Collateral [Line Items]
Restricted investments and bond collateral	13,063	13,062
Reclamation Bond Collateral | Coal Segment | Kemmerer Mine
Restricted Investments and Bond Collateral [Line Items]
Restricted investments and bond collateral	24,702	24,702
Reclamation Bond Collateral | Coal Segment | Absaloka Mine
Restricted Investments and Bond Collateral [Line Items]
Restricted investments and bond collateral	14,532	14,507
Reclamation Bond Collateral | Coal Segment | Rosebud Mine
Restricted Investments and Bond Collateral [Line Items]
Restricted investments and bond collateral	12,495	12,495
Reclamation Bond Collateral | Coal Segment | Beulah Mine
Restricted Investments and Bond Collateral [Line Items]
Restricted investments and bond collateral	1,270	1,270
Letter of Credit Account | Power Segment
Restricted Investments and Bond Collateral [Line Items]
Restricted investments and bond collateral	5,993	5,990
Postretirement Medical Benefit Bonds | Corporate Segment
Restricted Investments and Bond Collateral [Line Items]
Restricted investments and bond collateral	8,972	8,593
Workers' Compensation Bonds | Corporate Segment
Restricted Investments and Bond Collateral [Line Items]
Restricted investments and bond collateral	$ 6,609	$ 6,590