SUPPLEMENTAL CONSOLIDATING FINANCIAL INFORMATION - Supplemental Consolidating Balance Sheet Information (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Current assets:
Cash and cash equivalents	$ 40,852	$ 31,610	$ 50,107	$ 30,783	$ 30,783
Receivables:
Trade	60,218	60,037
Contractual third-party reclamation receivables	10,540	10,207
Intercompany receivable/payable	0	0
Other	2,968	3,220
Total receivables	73,726	73,464
Inventories	40,421	37,734
Other current assets	20,133	16,504
Total current assets	175,132	159,312
Property, plant and equipment:
Land and mineral rights	264,489	261,741
Plant and equipment	638,903	635,720
Gross property, plant and equipment	903,392	897,461
Less accumulated depreciation, depletion and amortization	399,344	384,621
Net property, plant and equipment	504,048	512,840
Advanced coal royalties	4,262	4,316
Reclamation deposits	73,100	72,718
Restricted investments and bond collateral	87,636	87,209
Contractual third-party reclamation receivables	84,089	84,158
Intangible assets	2,784	3,203
Investment in subsidiaries	0	0
Other assets	11,964	12,359
Total Assets	943,015	936,115	955,002
Current liabilities
Current installments of long-term debt	23,294	23,791
Accounts payable and accrued expenses:
Trade	57,294	52,093
Production taxes	38,154	33,228
Workers' compensation	815	820
Postretirement medical benefits	14,068	14,068
SERP	390	390
Deferred revenue	15,385	12,822
Asset retirement obligations	23,154	22,238	16,050
Other current liabilities	5,528	11,462
Total current liabilities	178,082	170,912
Long-term debt, less current installments	332,992	337,198
Workers' compensation, less current portion	8,524	8,710
Excess of black lung benefit obligation over trust assets	8,439	8,356
Postretirement medical benefits, less current portion	320,759	319,775
Pension and SERP obligations, less current portion	54,251	54,250
Deferred revenue, less current portion	54,488	56,891
Asset retirement obligations, less current portion	240,335	241,609	242,646
Intangible liabilities	6,370	6,625
Other liabilities	25,305	18,020
Intercompany receivable/payable	0	0
Total liabilities	1,229,545	1,222,346
Shareholders' deficit:
Preferred stock	160	160
Common stock	35,912	35,502
Other paid-in capital	132,828	130,852
Accumulated other comprehensive loss	(146,603)	(148,345)
Accumulated earnings (deficit)	(292,452)	(289,727)
Total Westmoreland Coal Company shareholders' deficit	(270,155)	(271,558)
Noncontrolling interest	(16,375)	(14,673)
Total equity (deficit)	(286,530)	(286,231)
Total Liabilities and Shareholders' Deficit	943,015	936,115
Parent/ Issuer
Current assets:
Cash and cash equivalents	10,971	14,836	32,394	26,141
Receivables:
Trade	0	0
Contractual third-party reclamation receivables	0	0
Intercompany receivable/payable	(5,940)	(8,002)
Other	28	77
Total receivables	(5,912)	(7,925)
Inventories	0	0
Other current assets	6,249	739
Total current assets	11,308	7,650
Property, plant and equipment:
Land and mineral rights	0	0
Plant and equipment	3,443	3,198
Gross property, plant and equipment	3,443	3,198
Less accumulated depreciation, depletion and amortization	2,457	(2,364)
Net property, plant and equipment	986	834
Advanced coal royalties	0	0
Reclamation deposits	0	0
Restricted investments and bond collateral	15,581	15,183
Contractual third-party reclamation receivables	0	0
Intangible assets	0	0
Investment in subsidiaries	241,373	248,565
Other assets	9,878	10,267
Total Assets	279,126	282,499
Current liabilities
Current installments of long-term debt	(1,594)	(1,548)
Accounts payable and accrued expenses:
Trade	3,369	4,707
Production taxes	0	0
Workers' compensation	815	820
Postretirement medical benefits	12,494	12,494
SERP	390	390
Deferred revenue	0	0
Asset retirement obligations	0	0
Other current liabilities	4,537	11,312
Total current liabilities	20,011	28,175
Long-term debt, less current installments	245,878	245,456
Workers' compensation, less current portion	8,524	8,710
Excess of black lung benefit obligation over trust assets	8,439	8,356
Postretirement medical benefits, less current portion	223,448	224,336
Pension and SERP obligations, less current portion	29,043	29,265
Deferred revenue, less current portion	0	0
Asset retirement obligations, less current portion	0	0
Intangible liabilities	0	0
Other liabilities	6,030	701
Intercompany receivable/payable	24,281	23,731
Total liabilities	565,654	568,730
Shareholders' deficit:
Preferred stock	160	160
Common stock	35,912	35,502
Other paid-in capital	132,828	130,852
Accumulated other comprehensive loss	(146,603)	(148,345)
Accumulated earnings (deficit)	(292,450)	(289,727)
Total Westmoreland Coal Company shareholders' deficit	(270,153)	(271,558)
Noncontrolling interest	(16,375)	(14,673)
Total equity (deficit)	(286,528)	(286,231)
Total Liabilities and Shareholders' Deficit	279,126	282,499
Co-Issuer
Current assets:
Cash and cash equivalents	4,807	4,545	2,487	6
Receivables:
Trade	10,133	13,018
Contractual third-party reclamation receivables	0	0
Intercompany receivable/payable	0	0
Other	0	0
Total receivables	10,133	13,018
Inventories	5,228	3,047
Other current assets	298	298
Total current assets	20,466	20,908
Property, plant and equipment:
Land and mineral rights	1,395	1,395
Plant and equipment	219,948	219,857
Gross property, plant and equipment	221,343	221,252
Less accumulated depreciation, depletion and amortization	64,005	(61,474)
Net property, plant and equipment	157,338	159,778
Advanced coal royalties	0	0
Reclamation deposits	0	0
Restricted investments and bond collateral	5,993	5,990
Contractual third-party reclamation receivables	0	0
Intangible assets	2,513	2,923
Investment in subsidiaries	0	0
Other assets	0	0
Total Assets	186,310	189,599
Current liabilities
Current installments of long-term debt	0	0
Accounts payable and accrued expenses:
Trade	10,955	4,978
Production taxes	407	3
Workers' compensation	0	0
Postretirement medical benefits	0	0
SERP	0	0
Deferred revenue	9,105	8,788
Asset retirement obligations	0	0
Other current liabilities	0	0
Total current liabilities	20,467	13,769
Long-term debt, less current installments	0	0
Workers' compensation, less current portion	0	0
Excess of black lung benefit obligation over trust assets	0	0
Postretirement medical benefits, less current portion	0	0
Pension and SERP obligations, less current portion	287	289
Deferred revenue, less current portion	47,995	50,239
Asset retirement obligations, less current portion	847	829
Intangible liabilities	6,370	6,625
Other liabilities	0	0
Intercompany receivable/payable	0	0
Total liabilities	75,966	71,751
Shareholders' deficit:
Preferred stock	0	0
Common stock	5	5
Other paid-in capital	52,813	52,807
Accumulated other comprehensive loss	(366)	(372)
Accumulated earnings (deficit)	57,892	65,408
Total Westmoreland Coal Company shareholders' deficit	110,344	117,848
Noncontrolling interest	0	0
Total equity (deficit)	110,344	117,848
Total Liabilities and Shareholders' Deficit	186,310	189,599
Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	6,698	5,362	2,696	143
Receivables:
Trade	14,506	13,428
Contractual third-party reclamation receivables	56	56
Intercompany receivable/payable	3,944	5,667
Other	18,195	16,806
Total receivables	36,701	35,957
Inventories	17,291	16,538
Other current assets	8,228	5,550
Total current assets	68,918	63,407
Property, plant and equipment:
Land and mineral rights	94,337	91,741
Plant and equipment	216,989	215,751
Gross property, plant and equipment	311,326	307,492
Less accumulated depreciation, depletion and amortization	113,261	(108,151)
Net property, plant and equipment	198,065	199,341
Advanced coal royalties	500	500
Reclamation deposits	0	0
Restricted investments and bond collateral	39,233	39,208
Contractual third-party reclamation receivables	339	327
Intangible assets	0	0
Investment in subsidiaries	(792)	(792)
Other assets	622	635
Total Assets	306,885	302,626
Current liabilities
Current installments of long-term debt	1,578	1,939
Accounts payable and accrued expenses:
Trade	12,710	15,163
Production taxes	12,332	10,014
Workers' compensation	0	0
Postretirement medical benefits	87	87
SERP	0	0
Deferred revenue	4,700	2,997
Asset retirement obligations	3,524	3,519
Other current liabilities	3	10
Total current liabilities	34,934	33,729
Long-term debt, less current installments	2,113	2,473
Workers' compensation, less current portion	0	0
Excess of black lung benefit obligation over trust assets	0	0
Postretirement medical benefits, less current portion	57,459	55,981
Pension and SERP obligations, less current portion	19,523	19,346
Deferred revenue, less current portion	0	0
Asset retirement obligations, less current portion	40,918	40,063
Intangible liabilities	0	0
Other liabilities	17,396	15,677
Intercompany receivable/payable	(8,840)	(7,972)
Total liabilities	163,503	159,297
Shareholders' deficit:
Preferred stock	0	0
Common stock	110	110
Other paid-in capital	94,632	93,456
Accumulated other comprehensive loss	(4,986)	(4,987)
Accumulated earnings (deficit)	53,626	54,750
Total Westmoreland Coal Company shareholders' deficit	143,382	143,329
Noncontrolling interest	0	0
Total equity (deficit)	143,382	143,329
Total Liabilities and Shareholders' Deficit	306,885	302,626
Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	18,376	6,867	12,530	4,493
Receivables:
Trade	35,579	33,591
Contractual third-party reclamation receivables	10,484	10,151
Intercompany receivable/payable	(30,992)	(30,641)
Other	1,071	1,182
Total receivables	16,142	14,283
Inventories	17,902	18,149
Other current assets	5,358	9,917
Total current assets	57,778	49,216
Property, plant and equipment:
Land and mineral rights	168,757	168,605
Plant and equipment	198,523	196,914
Gross property, plant and equipment	367,280	365,519
Less accumulated depreciation, depletion and amortization	219,621	(212,632)
Net property, plant and equipment	147,659	152,887
Advanced coal royalties	3,762	3,816
Reclamation deposits	73,100	72,718
Restricted investments and bond collateral	26,829	26,828
Contractual third-party reclamation receivables	83,750	83,831
Intangible assets	271	280
Investment in subsidiaries	3,770	3,770
Other assets	3,464	3,457
Total Assets	400,383	396,803
Current liabilities
Current installments of long-term debt	23,310	23,400
Accounts payable and accrued expenses:
Trade	46,577	42,085
Production taxes	25,415	23,211
Workers' compensation	0	0
Postretirement medical benefits	1,487	1,487
SERP	0	0
Deferred revenue	1,580	1,037
Asset retirement obligations	19,630	18,719
Other current liabilities	997	144
Total current liabilities	118,996	110,083
Long-term debt, less current installments	87,001	91,269
Workers' compensation, less current portion	0	0
Excess of black lung benefit obligation over trust assets	0	0
Postretirement medical benefits, less current portion	39,852	39,458
Pension and SERP obligations, less current portion	5,398	5,350
Deferred revenue, less current portion	6,493	6,652
Asset retirement obligations, less current portion	198,570	200,717
Intangible liabilities	0	0
Other liabilities	1,879	1,642
Intercompany receivable/payable	17,730	35,787
Total liabilities	475,919	490,958
Shareholders' deficit:
Preferred stock	0	0
Common stock	132	132
Other paid-in capital	63,046	62,539
Accumulated other comprehensive loss	(24,087)	(24,492)
Accumulated earnings (deficit)	(114,627)	(132,334)
Total Westmoreland Coal Company shareholders' deficit	(75,536)	(94,155)
Noncontrolling interest	0	0
Total equity (deficit)	(75,536)	(94,155)
Total Liabilities and Shareholders' Deficit	400,383	396,803
Consolidating Adjustments
Current assets:
Cash and cash equivalents	0	0	0	0
Receivables:
Trade	0	0
Contractual third-party reclamation receivables	0	0
Intercompany receivable/payable	32,988	32,976
Other	(16,326)	(14,845)
Total receivables	16,662	18,131
Inventories	0	0
Other current assets	0	0
Total current assets	16,662	18,131
Property, plant and equipment:
Land and mineral rights	0	0
Plant and equipment	0	0
Gross property, plant and equipment	0	0
Less accumulated depreciation, depletion and amortization	0	0
Net property, plant and equipment	0	0
Advanced coal royalties	0	0
Reclamation deposits	0	0
Restricted investments and bond collateral	0	0
Contractual third-party reclamation receivables	0	0
Intangible assets	0	0
Investment in subsidiaries	(244,351)	(251,543)
Other assets	(2,000)	(2,000)
Total Assets	(229,689)	(235,412)
Current liabilities
Current installments of long-term debt	0	0
Accounts payable and accrued expenses:
Trade	(16,317)	(14,840)
Production taxes	0	0
Workers' compensation	0	0
Postretirement medical benefits	0	0
SERP	0	0
Deferred revenue	0	0
Asset retirement obligations	0	0
Other current liabilities	(9)	(4)
Total current liabilities	(16,326)	(14,844)
Long-term debt, less current installments	(2,000)	(2,000)
Workers' compensation, less current portion	0	0
Excess of black lung benefit obligation over trust assets	0	0
Postretirement medical benefits, less current portion	0	0
Pension and SERP obligations, less current portion	0	0
Deferred revenue, less current portion	0	0
Asset retirement obligations, less current portion	0	0
Intangible liabilities	0	0
Other liabilities	0	0
Intercompany receivable/payable	(33,171)	(51,546)
Total liabilities	(51,497)	(68,390)
Shareholders' deficit:
Preferred stock	0	0
Common stock	(247)	(247)
Other paid-in capital	(210,491)	(208,802)
Accumulated other comprehensive loss	29,439	29,851
Accumulated earnings (deficit)	3,107	12,176
Total Westmoreland Coal Company shareholders' deficit	(178,192)	(167,022)
Noncontrolling interest	0	0
Total equity (deficit)	(178,192)	(167,022)
Total Liabilities and Shareholders' Deficit	$ (229,689)	$ (235,412)