COMMITMENTS AND CONTINGENCIES - UNCONDITIONAL PURCHASE OBLIGATIONS AND PURCHASE ORDER COMMITMENTS (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014
Minimum future commitments for purchase obligations
Total Amounts Committed	$ 1,286.2
2014	86.0
2015	136.9
2016	104.3
2017	116.4
2018	118.2
Later Years	724.4
Purchase orders	445.8
Purchased power | Electric Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	892.9
2014	37.8
2015	49.0
2016	42.3
2017	52.8
2018	55.8
Later Years	655.2
Coal supply and transportation | Electric Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	127.9
2014	26.1
2015	42.8
2016	18.5
2017	20.6
2018	19.9
Later Years	0
Natural gas utility supply and transportation | Natural Gas Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	265.4
2014	22.1
2015	45.1
2016	43.5
2017	43.0
2018	42.5
Later Years	$ 69.2