EMPLOYEE BENEFIT PLANS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Components of Net Periodic Benefit Cost
Pension Costs Related to Retail Electric Operations and Natural Gas Operations Deferred as Regulatory Assets	$ 3.7	$ 2.3
Pension Benefits
Components of Net Periodic Benefit Cost
Service cost	4.8	4.6
Interest cost	10.7	11.1
Expected return on assets	(14.8)	(16.2)
Other Comprehensive Income (Loss), Amortization, Pension and Other Postretirement Benefit Plans, Net Prior Service Cost (Credit) Recognized in Net Periodic Benefit Cost, before Tax	1.7	1.8
Defined Benefit Plan, Amortization of Transition Obligations (Assets)	0	0
Defined Benefit Plan, Actuarial Net (Gains) Losses	4.7	3.0
Defined Benefit Plan, Net Periodic Benefit Cost	7.1	4.3
Other Postretirement Benefits
Components of Net Periodic Benefit Cost
Service cost	1.3	1.1
Interest cost	3.0	3.0
Expected return on assets	0	0
Other Comprehensive Income (Loss), Amortization, Pension and Other Postretirement Benefit Plans, Net Prior Service Cost (Credit) Recognized in Net Periodic Benefit Cost, before Tax	0.2	0.3
Defined Benefit Plan, Amortization of Transition Obligations (Assets)	0.2	0.2
Defined Benefit Plan, Actuarial Net (Gains) Losses	0.2	0.1
Defined Benefit Plan, Net Periodic Benefit Cost	$ 4.9	$ 4.7