ACQUISITION - Summary of purchase consideration and allocation of purchase consideration (Details) (USD $)	3 Months Ended	9 Months Ended				0 Months Ended			3 Months Ended
	Sep. 30, 2014	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	Jun. 30, 2014 Canadian Acquisition	Apr. 28, 2014 Canadian Acquisition
Purchase Price:
Payments to Acquire Business, Initial Payment						$ 282,800,000		$ 282,800,000
Other Payments to Acquire Businesses						39,800,000	39,800,000
Cash paid		322,637,000	0			322,600,000	39,800,000	282,800,000	322,600,000
Assets:
Cash and cash equivalents	26,200,000	26,200,000		26,200,000	26,200,000
Trade receivables	49,900,000	49,900,000		49,900,000
Inventories - materials and supplies	52,000,000	52,000,000		52,000,000
Inventories - coal	79,800,000	79,800,000		79,800,000
Leases and loans receivable	11,200,000	11,200,000		11,200,000
Deferred tax assets	1,000,000	1,000,000		6,700,000
Deferred tax assets, Adjustment	(5,700,000)
Other current assets	3,400,000	3,400,000		3,400,000
Total current assets	223,500,000	223,500,000		229,200,000
Total current assets, Adjustment	(5,700,000)
Land and mineral rights	157,000,000	157,000,000		151,300,000
Land and mineral rights, Adjustment	5,700,000
Plant and equipment	110,800,000	110,800,000		139,600,000
Plant and equipment, Adjustment	(28,800,000)
Leases and loans receivable	79,100,000	79,100,000		83,800,000
Leases and loans receivable, Adjustment	(4,700,000)
Contractual third-party reclamation receivables, less current portion	6,800,000	6,800,000		6,800,000
Investment in joint venture	32,100,000	32,100,000		32,100,000
Intangible assets, Adjustment	37,000,000
Intangible assets	37,000,000	37,000,000
Other assets	8,600,000	8,600,000		10,300,000
Other assets, adjustment	(1,700,000)
Total Assets	654,900,000	654,900,000		653,100,000
Total assets, Adjustment	1,800,000
Liabilities:
Current installments of long-term debt	(36,300,000)	(36,300,000)		(36,300,000)
Trade payables and other accrued liabilities	(100,200,000)	(100,200,000)		(93,700,000)
Trade payables and other accrued liabilities, Adjustment	(6,500,000)
Asset retirement obligations	(9,700,000)	(9,700,000)		(9,700,000)
Total current liabilities	(146,200,000)	(146,200,000)		(139,700,000)
Total current liabilities, Adjustment	(6,500,000)
Long-term debt, less current installments	(86,300,000)	(86,300,000)		(86,300,000)
Asset retirement obligations, less current portion	(92,400,000)	(92,400,000)		(92,400,000)
Deferred tax liabilities	(7,400,000)	(7,400,000)		(12,100,000)
Deferred tax liabilities, Adjustment	4,700,000
Total Liabilities	(332,300,000)	(332,300,000)		(330,500,000)						(332,300,000)
Total liabilities, Adjustment	(1,800,000)
Net fair value	$ 322,600,000	$ 322,600,000		$ 322,600,000