ACCUMULATED OTHER COMPREHENSIVE LOSS RECLASSIFICATIONS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Amount reclassified out of AOCI
Operating and maintenance expense	$ 288.7	$ 249.0	$ 583.8	$ 508.3
Nonregulated revenues	(421.6)	(276.0)	(976.0)	(552.9)
Interest expense	28.6	29.7	57.9	60.1
Total before taxes	7.2	(81.3)	(284.6)	(227.5)
Tax expense	3.3	(29.6)	(106.3)	(77.0)
Net of tax	(3.9)	51.7	178.3	150.5
Regulated Operations
Amount reclassified out of AOCI
Interest expense	20.4	20.4	42.2	41.6
Tax expense	(23.4)	(13.5)	(111.1)	(82.7)
Net of tax	39.6	23.6	172.1	140.6
Amount reclassified
Amount reclassified out of AOCI
Net of tax	(2.1)		(3.6)
Losses on cash flow hedges | Amount reclassified
Amount reclassified out of AOCI
Total before taxes	2.4		3.9
Tax expense	0.9		1.5
Net of tax	(1.5)		(2.4)
Losses on cash flow hedges | Amount reclassified | Commodity derivative contracts | Regulated Operations
Amount reclassified out of AOCI
Operating and maintenance expense	0		0.2
Losses on cash flow hedges | Amount reclassified | Commodity derivative contracts | Nonutility and Nonregulated Operations
Amount reclassified out of AOCI
Nonregulated revenues	2.1		3.2
Losses on cash flow hedges | Amount reclassified | Interest rate hedges
Amount reclassified out of AOCI
Interest expense	0.3		0.5
Defined benefit plans | Amount reclassified
Amount reclassified out of AOCI
Amortization of prior service costs	0		(0.1)
Amortization of net actuarial loss	1.0		2.1
Total before taxes	1.0		2.0
Tax expense	0.4		0.8
Net of tax	$ (0.6)		$ (1.2)