WHITE OAK TRANSACTIONS (Details) (White Oak, USD $) In Millions, unless otherwise specified	0 Months Ended	3 Months Ended		6 Months Ended		15 Months Ended	21 Months Ended	6 Months Ended		0 Months Ended	6 Months Ended	15 Months Ended	0 Months Ended	6 Months Ended
	Sep. 22, 2011	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013	Jun. 30, 2013 Minimum	Jun. 30, 2013 Maximum	Sep. 22, 2011 Reserve Acquisition	Jun. 30, 2013 Reserve Acquisition	Dec. 31, 2012 Reserve Acquisition	Sep. 22, 2011 Reserve Acquisition, rights purchased on Transaction Date T	Jun. 30, 2013 Reserve Acquisition, rights purchased on Transaction Date T	Jun. 30, 2013 Reserve Acquisition, rights purchased during first half of 2013 T	Jun. 30, 2013 Services Agreement
White oak transactions
Amount funded by partnership	$ 69.5						$ 190.3
Total expected funding by partnership								300.5	425.5
Expected period of funding				1 year 6 months
Coal reserves, rights purchased (in tons)													204,900,000		66,500,000
Coal reserves developed for future mining (in tons)														105,200,000	34,300,000
Payment for acquisition of coal reserves and other assets										33.8	18.9
Payment for development of acquired coal reserves												51.6
Commitment for additional coal reserve acquisitions											35.7
Purchase of equity investment	35.7			47.5		66.8
Voting Interest(as a percent)		20.00%		20.00%			20.00%
Equity method investment, net		150.0		150.0			150.0
Distributions from White Oak							0
Allocated losses		5.9	4.6	10.1	8.6
Additional commitment for construction loan																10.5
Term of construction loan																20 years
Construction Loan utilized																$ 5.5