BASIS OF PRESENTATION - Balance Sheet Impact (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cash and cash equivalents	$ 63,187	$ 103,247	$ 57,620
Trade	78,504	89,311
Other	13,741	17,697
Total receivables	92,245	107,008
Inventories	137,240	106,795
Unbilled revenues	57,560	63,874
Other current assets	26,539	11,517
Total current assets	376,771	392,441
Gross property, plant and equipment	1,239,701	1,665,740
Less accumulated depreciation, depletion and amortization	657,109	923,905
Net property, plant and equipment	582,592	741,835
Advanced coal royalties	12,930	21,404
Restricted investments, reclamation deposits and bond collateral	205,812	200,194
Unbilled revenues, less current portion	218,892	225,245
Investment in joint venture	24,716	27,763
Other assets	35,882	55,036
Total assets	1,457,595	1,663,918	$ 1,670,219
Current installments of long-term debt	1,071,533	983,427
Trade and other accrued liabilities	106,125	121,489
Interest payable	26,042	22,840
Production taxes	40,915	41,688
Postretirement medical benefits	14,734	14,734
Deferred revenue	6,120	3,201
Asset retirement obligations	50,110	48,429
Other current liabilities	8,164	9,401
Total current liabilities	1,323,743	1,245,209
Long-term debt, less current installments	15,416	64,980
Postretirement medical benefits, less current portion	319,583	317,407
Pension and SERP obligations, less current portion	42,827	43,585
Deferred revenue, less current portion		0
Asset retirement obligations, less current portion	412,613	426,038
Other liabilities	29,598	31,477
Total liabilities	2,143,780	2,128,696
Common stock of $0.01 par value: Authorized 30,000,000 shares; Issued and outstanding 18,771,643 shares at December 31, 2017	188	188
Other paid-in capital	252,354	250,494
Accumulated other comprehensive loss	(170,289)	(158,699)
Accumulated deficit	(757,949)	(552,263)
Total Westmoreland Coal Company shareholders’ deficit	(675,696)	(460,280)
Noncontrolling interests in consolidated subsidiaries	(10,489)	(4,498)
Total deficit	(686,185)	(464,778)
Total Liabilities and Shareholders' Deficit	$ 1,457,595	1,663,918
Adjustments for New Revenue Standard | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cash and cash equivalents		0
Trade		0
Other		0
Total receivables		0
Inventories		0
Unbilled revenues		49,574
Other current assets		0
Total current assets		49,574
Gross property, plant and equipment		0
Less accumulated depreciation, depletion and amortization		0
Net property, plant and equipment		0
Advanced coal royalties		0
Restricted investments, reclamation deposits and bond collateral		0
Unbilled revenues, less current portion		225,245
Investment in joint venture		0
Other assets		0
Total assets		274,819
Current installments of long-term debt		0
Trade and other accrued liabilities		0
Interest payable		0
Production taxes		0
Postretirement medical benefits		0
Deferred revenue		(1,867)
Asset retirement obligations		0
Other current liabilities		0
Total current liabilities		(1,867)
Long-term debt, less current installments		0
Postretirement medical benefits, less current portion		0
Pension and SERP obligations, less current portion		0
Deferred revenue, less current portion		(1,984)
Asset retirement obligations, less current portion		0
Other liabilities		0
Total liabilities		(3,851)
Common stock of $0.01 par value: Authorized 30,000,000 shares; Issued and outstanding 18,771,643 shares at December 31, 2017		0
Other paid-in capital		0
Accumulated other comprehensive loss		1,826
Accumulated deficit		276,844
Total Westmoreland Coal Company shareholders’ deficit		278,670
Noncontrolling interests in consolidated subsidiaries		0
Total deficit		278,670
Total Liabilities and Shareholders' Deficit		274,819
Adjustments for New Revenue Standard | Accounting Standards Update 2014-09, Revised Recognition Period
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cash and cash equivalents		0
Trade		(14,300)
Other		0
Total receivables		(14,300)
Inventories		0
Unbilled revenues		14,300
Other current assets		0
Total current assets		0
Gross property, plant and equipment		0
Less accumulated depreciation, depletion and amortization		0
Net property, plant and equipment		0
Advanced coal royalties		0
Restricted investments, reclamation deposits and bond collateral		0
Unbilled revenues, less current portion		0
Investment in joint venture		0
Other assets		0
Total assets		0
Current installments of long-term debt		0
Trade and other accrued liabilities		0
Interest payable		0
Production taxes		0
Postretirement medical benefits		0
Deferred revenue		0
Asset retirement obligations		0
Other current liabilities		0
Total current liabilities		0
Long-term debt, less current installments		0
Postretirement medical benefits, less current portion		0
Pension and SERP obligations, less current portion		0
Deferred revenue, less current portion		0
Asset retirement obligations, less current portion		0
Other liabilities		0
Total liabilities		0
Common stock of $0.01 par value: Authorized 30,000,000 shares; Issued and outstanding 18,771,643 shares at December 31, 2017		0
Other paid-in capital		0
Accumulated other comprehensive loss		0
Accumulated deficit		0
Total Westmoreland Coal Company shareholders’ deficit		0
Noncontrolling interests in consolidated subsidiaries		0
Total deficit		0
Total Liabilities and Shareholders' Deficit		0
As Reported
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cash and cash equivalents		103,247
Trade		103,611
Other		17,697
Total receivables		121,308
Inventories		106,795
Unbilled revenues		0
Other current assets		11,517
Total current assets		342,867
Gross property, plant and equipment		1,665,740
Less accumulated depreciation, depletion and amortization		923,905
Net property, plant and equipment		741,835
Advanced coal royalties		21,404
Restricted investments, reclamation deposits and bond collateral		200,194
Unbilled revenues, less current portion		0
Investment in joint venture		27,763
Other assets		55,036
Total assets		1,389,099
Current installments of long-term debt		983,427
Trade and other accrued liabilities		121,489
Interest payable		22,840
Production taxes		41,688
Postretirement medical benefits		14,734
Deferred revenue		5,068
Asset retirement obligations		48,429
Other current liabilities		9,401
Total current liabilities		1,247,076
Long-term debt, less current installments		64,980
Postretirement medical benefits, less current portion		317,407
Pension and SERP obligations, less current portion		43,585
Deferred revenue, less current portion		1,984
Asset retirement obligations, less current portion		426,038
Other liabilities		31,477
Total liabilities		2,132,547
Common stock of $0.01 par value: Authorized 30,000,000 shares; Issued and outstanding 18,771,643 shares at December 31, 2017		188
Other paid-in capital		250,494
Accumulated other comprehensive loss		(160,525)
Accumulated deficit		(829,107)
Total Westmoreland Coal Company shareholders’ deficit		(738,950)
Noncontrolling interests in consolidated subsidiaries		(4,498)
Total deficit		(743,448)
Total Liabilities and Shareholders' Deficit		$ 1,389,099