ACQUISITION - Narrative (Details) (USD $)	0 Months Ended	3 Months Ended		6 Months Ended			0 Months Ended			2 Months Ended	3 Months Ended	6 Months Ended								6 Months Ended
	Apr. 28, 2014	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Apr. 28, 2014	Jun. 25, 2014 Canadian Acquisition	Apr. 28, 2014 Canadian Acquisition mines	Dec. 24, 2013 Canadian Acquisition	Jun. 30, 2014 Canadian Acquisition	Jun. 30, 2014 Canadian Acquisition	Jun. 30, 2014 Canadian Acquisition mines T	Apr. 28, 2014 Canadian Acquisition	Dec. 31, 2013 Canadian Acquisition T	Jun. 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	Jun. 30, 2014 Cost of Sales - Inventory Canadian Acquisition	Jun. 30, 2014 Selling and Administrative Costs Canadian Acquisition	Jun. 30, 2014 Foreign Exchange Canadian Acquisition	Jun. 30, 2014 Interest Expense Canadian Acquisition	Jun. 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		$ 39,800,000	$ 282,800,000	$ 283,000,000		$ 322,600,000
Assumed liabilities						330,500,000							330,500,000
Aggregate principal amount															675,500,000	425,000,000
Stated interest rate (percent)															10.75%	10.75%	8.02%
Acquisition related costs												31,600,000								13,600,000	6,900,000	6,200,000	4,900,000
Maximum borrowing capacity																		60,000,000	60,000,000
Cash and cash equivalents						26,200,000
Payment of assumed liability	18,100,000
Net cash received upon acquisition						8,100,000
Revenues		287,956,000	162,499,000	468,159,000	323,947,000					116,000,000
Income (loss) from operations		$ (29,640,000)	$ 11,975,000	$ (21,588,000)	$ 17,710,000					$ (12,700,000)