INCOME TAXES (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Deferred income tax assets
Tax credit carryforwards	$ 113.5	$ 105.1
Price risk management	13.0	59.6
Other	98.5	76.5
Total deferred income tax assets	225.0	241.2
Valuation allowance	(8.2)	(7.2)
Net deferred income tax assets	216.8	234.0
Deferred income tax liabilities
Plant-related	1,373.8	1,215.7
Employee benefits	79.6	49.1
Regulatory deferrals	78.8	60.8
Other	43.5	47.9
Total deferred income tax liabilities	1,575.7	1,373.5
Balance sheet presentation
Current deferred income tax assets	31.4	64.3
Long-term deferred income tax liabilities	1,390.3	1,203.8
Net deferred income tax liabilities	1,358.9	1,139.5
Income taxes, additional disclosures
Deferred tax credit carryforwards of alternative minimum tax credits	73.9
Deferred tax credit carryforwards of general business credits	20.9
Deferred tax credit carryforwards of foreign tax credits	15.2
Deferred tax credit carryforwards of state tax credits	3.5
Deferred income tax assets reflecting federal operating loss carryforwards	45.4
Deferred income tax assets reflecting state operating loss carryforwards	16.4
Deferred tax assets expiring between 2020 and 2032
2020 through 2025	6.3
2026 through 2031	9.2
2032	46.3
Net regulatory asset for regulatory tax effects	51.6	42.1
Components of foreign and domestic income included in income before taxes
Domestic	560.8	443.9	363.4
Foreign	0	(0.1)	(0.1)
Income before taxes	560.8	443.8	363.3
Current provision
Federal	7.8	3.5	(44.2)
State	5.9	0.4	6.0
Foreign	0.1	(0.1)	(0.2)
Total current provision	13.8	3.8	(38.4)
Deferred provision
Federal	174.8	128.3	158.7
State	14.9	20.1	14.6
Foreign	0	0	0.1
Total deferred provision	189.7	148.4	173.4
Investment tax credits, net	8.4	5.2	(1.1)
Penalties	(0.1)	(0.3)	0.7
Unrecognized tax benefits	0.4	(3.0)	0.9
Interest	(1.4)	(4.3)	(2.2)
Total provision for income taxes related to continuing operations	210.8	149.8	133.3
Total provision for income taxes related to discontiued operations	(6.9)	(9.3)	1.1
Total	$ 203.9	$ 140.5	$ 134.4
Foreign carryforwards
Income taxes, additional disclosures
Carryback period of tax credits	1 year
Carryforward period of tax credits	10 years
State carryforwards
Income taxes, additional disclosures
Carryforward period of tax credits	5 years
Carryforward period of operating loss carryforwards	20 years
Federal carryforwards
Income taxes, additional disclosures
Carryback period of operating loss carryforwards	2 years
Carryforward period of operating loss carryforwards	20 years
General business credits
Income taxes, additional disclosures
Carryback period of tax credits	1 year
Carryforward period of tax credits	20 years