SEGMENTS OF BUSINESS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 segment	Sep. 30, 2013
Segment Reporting [Abstract]
Number of reportable segments			3
Segment reporting information
Revenues	$ 370.4	$ 371.9	$ 1,284.9	$ 1,173.1
Depreciation and amortization expense	28.7	27.6	84.6	78.8
Miscellaneous income	5.6	6.0	19.5	16.9
Interest expense	14.6	10.7	42.9	31.9
Provision (benefit) for income taxes	26.0	22.8	66.7	64.7
Preferred stock dividend requirements	(0.7)	(0.7)	(2.3)	(2.3)
Net income (loss) attributed to common shareholder	42.2	37.0	109.6	107.5
Intersegment revenues
Segment reporting information
Revenues	0	0	0	0
Reconciling Eliminations
Segment reporting information
Revenues	0	0	0	0
Depreciation and amortization expense	(0.1)	(0.1)	(0.4)	(0.4)
Miscellaneous income	0	0	0	0
Interest expense	0	0	0	0
Provision (benefit) for income taxes	0	0	0	0
Preferred stock dividend requirements	0	0	0	0
Net income (loss) attributed to common shareholder	0	0	0	0
Reconciling Eliminations | Intersegment revenues
Segment reporting information
Revenues	(3.7)	(4.6)	(11.5)	(9.4)
Utility Segments
Segment reporting information
Revenues	370.4	371.9	1,284.9	1,173.1
Depreciation and amortization expense	28.7	27.5	84.5	78.7
Miscellaneous income	2.1	2.8	8.5	6.6
Interest expense	14.0	10.0	41.3	30.2
Provision (benefit) for income taxes	25.0	21.8	63.7	61.8
Preferred stock dividend requirements	(0.7)	(0.7)	(2.3)	(2.3)
Net income (loss) attributed to common shareholder	40.1	35.3	102.8	101.5
Utility Segments | Intersegment revenues
Segment reporting information
Revenues	3.4	4.1	10.5	8.3
Utility Segments | Electric Utility
Segment reporting information
Revenues	327.4	331.7	940.2	946.0
Depreciation and amortization expense	24.6	23.6	72.3	66.9
Miscellaneous income	2.1	2.8	8.4	6.5
Interest expense	11.4	7.9	33.5	23.8
Provision (benefit) for income taxes	27.3	23.8	52.5	52.9
Preferred stock dividend requirements	(0.7)	(0.6)	(2.0)	(1.9)
Net income (loss) attributed to common shareholder	43.1	38.3	85.2	87.1
Utility Segments | Electric Utility | Intersegment revenues
Segment reporting information
Revenues	0	0	0	0
Utility Segments | Natural Gas Utility
Segment reporting information
Revenues	43.0	40.2	344.7	227.1
Depreciation and amortization expense	4.1	3.9	12.2	11.8
Miscellaneous income	0	0	0.1	0.1
Interest expense	2.6	2.1	7.8	6.4
Provision (benefit) for income taxes	(2.3)	(2.0)	11.2	8.9
Preferred stock dividend requirements	0	(0.1)	(0.3)	(0.4)
Net income (loss) attributed to common shareholder	(3.0)	(3.0)	17.6	14.4
Utility Segments | Natural Gas Utility | Intersegment revenues
Segment reporting information
Revenues	3.4	4.1	10.5	8.3
Nonutility Segments | Other
Segment reporting information
Revenues	0	0	0	0
Depreciation and amortization expense	0.1	0.2	0.5	0.5
Miscellaneous income	3.5	3.2	11.0	10.3
Interest expense	0.6	0.7	1.6	1.7
Provision (benefit) for income taxes	1.0	1.0	3.0	2.9
Preferred stock dividend requirements	0	0	0	0
Net income (loss) attributed to common shareholder	2.1	1.7	6.8	6.0
Nonutility Segments | Other | Intersegment revenues
Segment reporting information
Revenues	$ 0.3	$ 0.5	$ 1.0	$ 1.1