COMMITMENTS AND CONTINGENCIES - UNCONDITIONAL PURCHASE OBLIGATIONS AND PURCHASE ORDER COMMITMENTS (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014
Minimum future commitments for purchase obligations
Total Amounts Committed	$ 1,324.5
2014	46.8
2015	209.4
2016	106.8
2017	118.0
2018	119.2
Later Years	724.3
Purchase orders	384.8
Purchased power | Electric Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	944.0
2014	19.1
2015	118.9
2016	42.3
2017	52.8
2018	55.8
Later Years	655.1
Coal supply and transportation | Electric Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	124.9
2014	15.6
2015	45.1
2016	21.1
2017	22.2
2018	20.9
Later Years	0
Natural gas utility supply and transportation | Natural Gas Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	255.6
2014	12.1
2015	45.4
2016	43.4
2017	43.0
2018	42.5
Later Years	$ 69.2