ACQUISITION - Summary of purchase consideration and allocation of purchase consideration (Details) (USD $)	6 Months Ended			0 Months Ended			3 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Apr. 28, 2014	Jun. 25, 2014 Canadian Acquisition	Apr. 28, 2014 Canadian Acquisition	Dec. 24, 2013 Canadian Acquisition	Jun. 30, 2014 Canadian Acquisition	Apr. 28, 2014 Canadian Acquisition
Purchase Price:
Cash paid	$ 322,637,000	$ 0		$ 39,800,000	$ 282,800,000	$ 283,000,000	$ 322,600,000
Assets:
Cash and cash equivalents			26,200,000
Trade receivables			49,900,000
Inventories - materials and supplies			52,000,000
Inventories - coal			79,800,000
Leases and loans receivable			11,200,000
Deferred tax assets			6,700,000
Other current assets			3,400,000
Total current assets			229,200,000
Land and mineral rights			151,300,000
Plant and equipment			171,700,000
Leases and loans receivable			83,800,000
Contractual third-party reclamation receivables, less current portion			6,800,000
Other assets			10,300,000
Total Assets			653,100,000
Liabilities:
Current installments of long-term debt			(36,300,000)
Asset retirement obligations			(93,700,000)
Asset retirement obligations			(9,700,000)
Total current liabilities			(139,700,000)
Long-term debt, less current installments			(86,300,000)
Asset retirement obligations, less current portion			(92,400,000)
Deferred tax liabilities			(12,100,000)
Total Liabilities			(330,500,000)					(330,500,000)
Net fair value			$ 322,600,000