COMMITMENTS AND CONTINGENCIES (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013
Unconditional Purchase Obligations and Purchase Order Commitments
Total Amounts Committed	$ 1,192.9
2013	127.7
2014	124.8
2015	105.3
2016	74.0
2017	68.8
Later Years	692.3
Purchased power | Electric Utility
Unconditional Purchase Obligations and Purchase Order Commitments
Total Amounts Committed	796.6
2013	75.8
2014	34.8
2015	31.3
2016	28.1
2017	27.0
Later Years	599.6
Coal supply and transportation | Electric Utility
Unconditional Purchase Obligations and Purchase Order Commitments
Total Amounts Committed	116.5
2013	29.4
2014	44.2
2015	31.5
2016	7.2
2017	4.2
Later Years	0
Natural gas utility supply and transportation | Natural Gas Utility
Unconditional Purchase Obligations and Purchase Order Commitments
Total Amounts Committed	279.8
2013	22.5
2014	45.8
2015	42.5
2016	38.7
2017	37.6
Later Years	92.7
Purchase orders
Unconditional Purchase Obligations and Purchase Order Commitments
Supply contract obligation amount	$ 250.3