COMMITMENTS AND CONTINGENCIES (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Unconditional purchase obligations
Total Amounts Committed	$ 2,629.1
2013	770.4
2014	541.1
2015	235.6
2016	165.9
2017	117.1
Later Years	799.0
Natural gas supply and transportation | Natural Gas Utility
Unconditional purchase obligations
Total Amounts Committed	823.2
2013	124.9
2014	156.3
2015	137.9
2016	125.7
2017	86.9
Later Years	191.5
Purchased power | Electric Utility
Unconditional purchase obligations
Total Amounts Committed	852.6
2013	121.3
2014	37.7
2015	33.0
2016	29.4
2017	28.4
Later Years	602.8
Coal supply and transportation | Electric Utility
Unconditional purchase obligations
Total Amounts Committed	122.7
2013	44.4
2014	41.2
2015	30.4
2016	6.2
2017	0.5
Electricity and natural gas supply | Nonutility and Nonregulated Operations
Unconditional purchase obligations
Total Amounts Committed	830.6
2013	479.8
2014	305.9
2015	34.3
2016	4.6
2017	1.3
Later Years	4.7
Purchase orders
Purchase and sales commitments
Supply contract obligation amount	$ 713.5