COMMITMENTS AND CONTINGENCIES (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended			0 Months Ended	15 Months Ended	0 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Sep. 22, 2011 White Oak	Dec. 31, 2012 White Oak	Sep. 22, 2011 White Oak Minimum	Sep. 22, 2011 White Oak Maximum
Commitments And Contingencies
Rental expense	$ 5.1	$ 5.3	$ 7.2
Contractual commitments amount	96.9
Estimated cost of commitments to purchase coal	6.7
Amount funded by partnership				69.5	121.5
Expected funding by partnership						300.5	425.5
Expected period of funding						2 years	3 years
Aggregate maximum insurance limit	100.0
Insurance deductible	1.5
Waiting period	90 days
Overall aggregate deductible	$ 10.0