COMMITMENTS AND CONTINGENCIES - UNCONDITIONAL PURCHASE OBLIGATIONS (Details) - Regulated operations $ in Millions	Jun. 30, 2015 USD ($)
Minimum future commitments for purchase obligations
Total Amounts Committed	$ 1,596.0
2015	183.4
2016	291.3
2017	224.5
2018	170.4
2019	121.5
Later Years	604.9
Natural gas utility supply and transportation | Wisconsin
Minimum future commitments for purchase obligations
Total Amounts Committed	218.9
2015	20.7
2016	43.8
2017	42.9
2018	42.4
2019	27.1
Later Years	42.0
Natural gas utility supply and transportation | Illinois
Minimum future commitments for purchase obligations
Total Amounts Committed	191.3
2015	39.5
2016	76.7
2017	49.2
2018	14.1
2019	4.0
Later Years	7.8
Natural gas utility supply and transportation | Other States
Minimum future commitments for purchase obligations
Total Amounts Committed	215.5
2015	26.2
2016	52.9
2017	43.4
2018	23.7
2019	21.2
Later Years	48.1
Purchased power | Wisconsin
Minimum future commitments for purchase obligations
Total Amounts Committed	816.3
2015	61.4
2016	78.9
2017	54.1
2018	56.8
2019	58.1
Later Years	507.0
Coal supply and transportation | Wisconsin
Minimum future commitments for purchase obligations
Total Amounts Committed	154.0
2015	35.6
2016	39.0
2017	34.9
2018	33.4
2019	11.1
Later Years	$ 0.0