RESTRICTED INVESTMENTS AND BOND COLLATERAL - Components of Restricted Investments and Bond Collateral (Detail) (USD $)	Dec. 31, 2014	Feb. 28, 2014	Dec. 31, 2013
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	$ 164,389,000		$ 75,233,000
Less current portion	0	(5,998,000)	(5,998,000)
Total restricted investments and bond collateral, less current portion	164,389,000		69,235,000
Coal Canada Segment [Member] | Prairie Mines and Royalty ULC [Member]
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	18,199,000		0
Coal Canada Segment [Member] | Coal Valley Resources Inc. [Member]
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	31,866,000		0
Westmoreland Resource Partners LP [Member] | Coal Westmoreland Resource Partners LP [Member]
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	10,634,000		0
Debt Reserve Account [Member] | Coal [Member]
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	0		13,067,000
Reclamation Bond Collateral [Member] | Coal [Member]
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	34,000,000		0
Reclamation Bond Collateral [Member] | Coal [Member] | Kemmerer Mine [Member]
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	25,282,000		24,966,000
Reclamation Bond Collateral [Member] | Coal [Member] | Absaloka Mine [Member]
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	11,781,000		11,653,000
Reclamation Bond Collateral [Member] | Coal [Member] | Rosebud Mine [Member]
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	3,145,000		3,145,000
Reclamation Bond Collateral [Member] | Coal [Member] | Beulah Mine [Member]
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	1,270,000		1,270,000
Letter of Credit Account [Member] | Power [Member]
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	0		5,998,000
Power Contract Collateral [Member] | Power [Member]
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	12,600,000		0
Postretirement Medical Benefit Bonds [Member] | Corporate [Member]
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	8,780,000		8,467,000
Workers' Compensation Bonds [Member] | Corporate [Member]
Restricted Investments and Bond Collateral [Line Items]
Total restricted investments and bond collateral	$ 6,832,000		$ 6,667,000