ACCUMULATED OTHER COMPREHENSIVE LOSS - RECLASSIFICATIONS (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Amount reclassified out of accumulated other comprehensive loss
Operating and maintenance expense	$ 1,199.7	$ 1,086.7	$ 943.0
Interest expense	154.8	127.4	118.9
Net of tax	(279.9)	(354.8)	(284.3)
Amount reclassified
Amount reclassified out of accumulated other comprehensive loss
Net of tax	1.6	3.8
Utility segments
Amount reclassified out of accumulated other comprehensive loss
Interest expense	101.8	86.6	83.2
Losses (gains) on cash flow hedges | Amount reclassified
Amount reclassified out of accumulated other comprehensive loss
Total before taxes	1.1	5.0
Tax expense	1.2	3.6
Net of tax	(0.1)	1.4
Losses (gains) on cash flow hedges | Commodity derivative contracts | Utility segments | Amount reclassified
Amount reclassified out of accumulated other comprehensive loss
Operating and maintenance expense	0	0.2
Losses (gains) on cash flow hedges | Commodity derivative contracts | Nonregulated segments | Amount reclassified
Amount reclassified out of accumulated other comprehensive loss
Discontinued operations	0	3.7
Losses (gains) on cash flow hedges | Interest rate hedges | Amount reclassified
Amount reclassified out of accumulated other comprehensive loss
Interest expense	1.1	1.1
Defined benefit plans | Amount reclassified
Amount reclassified out of accumulated other comprehensive loss
Amortization of prior service costs (credits)	(0.2)	4.3
Amortization of net actuarial losses (gains)	2.7	(0.2)
Total before taxes	2.5	4.1
Tax expense	0.8	1.7
Net of tax	$ 1.7	$ 2.4