LEASE OBLIGATIONS (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating and Capital Leased Assets [Line Items]
Operating lease expense	$ 25.8	$ 26.3	$ 24.2
Capital lease assets	471.7	406.0
Accumulated amortization of capital leases	184.5	110.6
Total minimum capital lease payments	371.7
Total minimum operating lease payments	95.5
North American Coal [Member]
Operating and Capital Leased Assets [Line Items]
Capital lease obligation	$ 75.9