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	$ 1,832.7
2014	92.4
2015	350.9
2016	231.7
2017	204.5
2018	153.7
Later Years	799.5
Purchase orders	1,043.8
Natural gas utility supply and transportation | Regulated operations | Natural Gas Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	763.8
2014	57.7
2015	186.9
2016	168.3
2017	129.5
2018	77.0
Later Years	144.4
Purchased power | Regulated operations | 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 | Regulated operations | 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