COMMITMENTS AND CONTINGENCIES - UNCONDITIONAL PURCHASE OBLIGATIONS AND PURCHASE ORDER COMMITMENTS (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014
Minimum future commitments for purchase obligations
Total Amounts Committed	$ 1,265.7
2014	137.0
2015	120.7
2016	92.1
2017	96.9
2018	94.7
Later Years	724.3
Purchase orders	487.8
Purchased power | Electric Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	909.8
2014	56.6
2015	48.8
2016	42.1
2017	52.5
2018	53.3
Later Years	656.5
Coal supply and transportation | Electric Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	94.6
2014	47.0
2015	28.5
2016	9.9
2017	5.9
2018	3.3
Later Years	0
Natural gas utility supply and transportation | Natural Gas Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	261.3
2014	33.4
2015	43.4
2016	40.1
2017	38.5
2018	38.1
Later Years	$ 67.8