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
Total deferred income tax assets	$ 3.5	$ 3.9
Deferred income tax liabilities
Plant-related	491.7	449.9
Employee benefits	81.0	54.9
Regulatory deferrals	44.4	26.2
Other	16.7	15.9
Total deferred income tax liabilities	633.8	546.9
Balance sheet presentation
Current deferred income tax liabilities - included in other current liabilities	10.8	4.0
Long-term deferred income tax liabilities	619.5	539.0
Total net deferred income tax liabilities	630.3	543.0
Income taxes, additional disclosures
Deferred tax credit carryforwards of alternative minimum tax credits	1.7
Deferred tax credit carryforwards of general business credits	1.1
Net regulatory asset for regulatory tax effects	25.9	17.4
Current provision
Federal	(1.3)	24.8	15.4
State	3.4	4.2	4.2
Total current provision	2.1	29.0	19.6
Deferred provision
Federal	71.5	27.8	46.1
State	8.3	5.9	7.9
Total deferred provision	79.8	33.7	54.0
Interest	0.3	0.1	0
Investment tax credits, net	(0.3)	(0.2)	(0.4)
Total provision for income taxes	$ 81.9	$ 62.6	$ 73.2
General business credits
Income taxes, additional disclosures
Carryback period of general business tax credits	1 year
Carryforward period of general business tax credits	20 years