ACQUISITION - Narrative (Details) (USD $)	0 Months Ended	3 Months Ended		9 Months Ended				0 Months Ended			3 Months Ended	5 Months Ended	9 Months Ended								9 Months Ended
	Apr. 28, 2014	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Jun. 30, 2014	Apr. 28, 2014	Sep. 30, 2014 Canadian Acquisition	Jun. 25, 2014 Canadian Acquisition	Apr. 28, 2014 Canadian Acquisition mines	Jun. 30, 2014 Canadian Acquisition	Sep. 30, 2014 Canadian Acquisition	Sep. 30, 2014 Canadian Acquisition mines T	Apr. 28, 2014 Canadian Acquisition	Dec. 31, 2013 Canadian Acquisition T	Sep. 30, 2014 Senior Notes	Apr. 28, 2014 Senior Notes Canadian Acquisition	Apr. 28, 2014 Term Debt Canadian Acquisition	Apr. 28, 2014 Revolving Credit Facility	Apr. 28, 2014 Revolving Credit Facility Canadian Acquisition	Sep. 30, 2014 Cost of Sales - Inventory Canadian Acquisition	Sep. 30, 2014 Selling and Administrative Costs Canadian Acquisition	Sep. 30, 2014 Foreign Exchange Canadian Acquisition	Sep. 30, 2014 Interest Expense Canadian Acquisition	Sep. 30, 2014 PMRU mines
Business Acquisition [Line Items]
Number of mines										6			6												5
Voting interests acquired (percent)														50.00%
Proven or probable coal reserves															649,000,000
Cash paid				$ 322,637,000	$ 0			$ 322,600,000	$ 39,800,000	$ 282,800,000	$ 322,600,000
Assumed liabilities		332,300,000		332,300,000		330,500,000								332,300,000
Aggregate principal amount																675,500,000	425,000,000
Stated interest rate (percent)																10.75%	10.75%	8.02%
Acquisition related costs													33,100,000								14,200,000	7,800,000	6,200,000	4,900,000
Maximum borrowing capacity																			60,000,000	60,000,000
Cash and cash equivalents		26,200,000		26,200,000		26,200,000	26,200,000
Payment of assumed liability	18,100,000
Net cash received upon acquisition							8,100,000
Intangible assets		37,000,000		37,000,000
Revenues		337,830,000	176,792,000	805,989,000	500,739,000							267,400,000
Income (loss) from operations		$ (29,432,000)	$ 8,536,000	$ (51,021,000)	$ 26,245,000							$ (14,200,000)