INCOME TAXES (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Deferred income tax assets
Total deferred income tax assets	4.4	$ 3.5
Deferred income tax liabilities
Plant-related	591.0	491.7
Employee benefits	83.9	81.0
Regulatory deferrals	42.4	44.4
Other	13.0	16.7
Total deferred income tax liabilities	730.3	633.8
Balance sheet presentation
Current deferred income tax liabilities - included in other current liabilities	3.8	10.8
Long-term deferred income tax liabilities	722.1	619.5
Total net deferred income tax liabilities	725.9	630.3
Income taxes, additional disclosures
Deferred tax credit carryforwards of alternative minimum tax credits	1.7
Deferred tax credit carryforwards of general business credits	1.9
Net regulatory asset for regulatory tax effects	32.7	$ 25.9
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