ACCUMULATED OTHER COMPREHENSIVE LOSS (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Amount reclassified out of AOCI
Operating and maintenance expense									$ 1,192.3	$ 1,031.3	$ 1,024.8
Nonregulated revenues									(2,209.0)	(1,252.9)	(1,391.4)
Interest expense									128.2	120.2	128.2
Total before taxes									(560.8)	(443.8)	(363.3)
Tax expense									(210.8)	(149.8)	(133.3)
Net of tax	(132.3)	(39.4)	3.9	(182.2)	(69.2)	(74.3)	(51.7)	(98.8)	(350.0)	(294.0)	(230.0)
Amount reclassified
Amount reclassified out of AOCI
Net of tax									3.8
Utility segments
Amount reclassified out of AOCI
Interest expense									86.6	83.2	90.2
Tax expense									(181.4)	(143.7)	(151.7)
Net of tax									(291.3)	(256.8)	(254.7)
Losses on cash flow hedges | Amount reclassified
Amount reclassified out of AOCI
Total before taxes									5.0
Tax expense									3.6
Net of tax									1.4
Losses on cash flow hedges | Commodity derivative contracts | Utility segments | Amount reclassified
Amount reclassified out of AOCI
Operating and maintenance expense									0.2
Losses on cash flow hedges | Commodity derivative contracts | Nonregulated segments | Amount reclassified
Amount reclassified out of AOCI
Nonregulated revenues									3.7
Losses on cash flow hedges | Interest rate hedges | Amount reclassified
Amount reclassified out of AOCI
Interest expense									1.1
Defined benefit plans | Amount reclassified
Amount reclassified out of AOCI
Amortization of prior service costs									4.3
Amortization of net actuarial gains									(0.2)
Total before taxes									4.1
Tax expense									1.7
Net of tax									$ 2.4