ACCUMULATED OTHER COMPREHENSIVE LOSS - RECLASSIFICATIONS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Amount reclassified out of AOCI
Operating and maintenance expense	$ 334.3	$ 288.7	$ 698.9	$ 583.8
Nonregulated revenues	(626.5)	(421.6)	(1,934.7)	(976.0)
Interest expense	38.7	28.6	77.8	57.9
Total before taxes	(16.3)	7.2	(259.3)	(284.6)
Tax expense	(8.2)	3.3	(98.0)	(106.3)
Net of tax	(8.1)	3.9	(161.3)	(178.3)
Amount reclassified
Amount reclassified out of AOCI
Net of tax	0.7	2.1	0.4	3.6
Utility segments
Amount reclassified out of AOCI
Interest expense	25.3	20.4	50.4	42.2
Tax expense	(12.9)	(23.4)	(106.5)	(111.1)
Net of tax	(21.1)	(39.6)	(165.8)	(172.1)
Losses on cash flow hedges | Amount reclassified
Amount reclassified out of AOCI
Total before taxes	0.2	2.4	0.5	3.9
Tax expense	0	0.9	0.9	1.5
Net of tax	0.2	1.5	(0.4)	2.4
Losses on cash flow hedges | Commodity derivative contracts | Utility segments | Amount reclassified
Amount reclassified out of AOCI
Operating and maintenance expense	0	0	0	0.2
Losses on cash flow hedges | Commodity derivative contracts | Nonregulated segments | Amount reclassified
Amount reclassified out of AOCI
Nonregulated revenues	0	2.1	0	3.2
Losses on cash flow hedges | Interest rate hedges | Amount reclassified
Amount reclassified out of AOCI
Interest expense	0.2	0.3	0.5	0.5
Defined benefit plans | Amount reclassified
Amount reclassified out of AOCI
Amortization of prior service credits	0	0	(0.1)	(0.1)
Amortization of net actuarial losses	0.7	1.0	1.4	2.1
Total before taxes	0.7	1.0	1.3	2.0
Tax expense	0.2	0.4	0.5	0.8
Net of tax	$ 0.5	$ 0.6	$ 0.8	$ 1.2