COMMITMENTS AND CONTINGENCIES (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013
Unconditional Purchase Obligations and Purchase Order Commitments
Total Amounts Committed	$ 1,176.2
2013	65.6
2014	152.3
2015	107.2
2016	77.2
2017	71.4
Later Years	702.5
Purchased power | Electric Utility
Unconditional Purchase Obligations and Purchase Order Commitments
Total Amounts Committed	783.8
2013	35.7
2014	61.3
2015	32.0
2016	28.2
2017	27.0
Later Years	599.6
Coal supply and transportation | Electric Utility
Unconditional Purchase Obligations and Purchase Order Commitments
Total Amounts Committed	110.6
2013	17.3
2014	44.4
2015	31.9
2016	9.5
2017	6.0
Later Years	1.5
Natural gas utility supply and transportation | Natural Gas Utility
Unconditional Purchase Obligations and Purchase Order Commitments
Total Amounts Committed	281.8
2013	12.6
2014	46.6
2015	43.3
2016	39.5
2017	38.4
Later Years	101.4
Purchase orders
Unconditional Purchase Obligations and Purchase Order Commitments
Commitments in the form of purchase orders issued to vendors	$ 365.0