ACCUMULATED OTHER COMPREHENSIVE LOSS - RECLASSIFICATIONS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Amount reclassified out of accumulated other comprehensive loss
Operating and maintenance expense	$ 289.8	$ 282.3	$ 988.7	$ 866.1
Nonregulated revenues	(562.8)	(522.8)	(2,497.5)	(1,498.8)
Interest expense	38.1	33.1	115.9	91.0
Total before taxes	(139.7)	(57.4)	(399.0)	(342.0)
Tax expense	(56.8)	(18.0)	(154.8)	(124.3)
Net of tax	(82.9)	(39.4)	(244.2)	(217.7)
Amount reclassified
Amount reclassified out of accumulated other comprehensive loss
Net of tax	0.5	0.9	0.9	4.5
Utility segments
Amount reclassified out of accumulated other comprehensive loss
Interest expense	25.4	21.5	75.8	63.7
Tax expense	(52.1)	(14.2)	(158.2)	(125.3)
Net of tax	(81.8)	(35.1)	(247.1)	(207.2)
Losses (gains) on cash flow hedges | Amount reclassified
Amount reclassified out of accumulated other comprehensive loss
Total before taxes	0.3	0.5	0.8	4.4
Tax expense	0.2	0.2	1.1	1.7
Net of tax	0.1	0.3	(0.3)	2.7
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	0	0.2
Losses (gains) on cash flow hedges | Commodity derivative contracts | Nonregulated segments | Amount reclassified
Amount reclassified out of accumulated other comprehensive loss
Nonregulated revenues	0	0.2	0	3.4
Losses (gains) on cash flow hedges | Interest rate hedges | Amount reclassified
Amount reclassified out of accumulated other comprehensive loss
Interest expense	0.3	0.3	0.8	0.8
Defined benefit plans | Amount reclassified
Amount reclassified out of accumulated other comprehensive loss
Amortization of prior service credits	0	(0.1)	(0.1)	(0.2)
Amortization of net actuarial losses	0.6	1.1	2.0	3.2
Total before taxes	0.6	1.0	1.9	3.0
Tax expense	0.2	0.4	0.7	1.2
Net of tax	$ 0.4	$ 0.6	$ 1.2	$ 1.8