SEGMENTS OF BUSINESS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 segment	Sep. 30, 2012
Segment Reporting [Abstract]
Number of reportable segments			3
Segment reporting information
External revenues	$ 371.9	$ 378.1	$ 1,173.1	$ 1,119.8
Intersegment revenues	0	0	0	0
Depreciation and amortization expense	27.6	24.1	78.8	72.0
Miscellaneous income	6.0	3.8	16.9	11.4
Interest expense	10.7	10.5	31.9	31.9
Provision for income taxes	22.8	14.4	64.7	48.1
Preferred stock dividend requirements	(0.7)	(0.7)	(2.3)	(2.3)
Net income (loss) attributed to common shareholder	37.0	42.5	107.5	107.2
Other
Segment reporting information
External revenues	0	0	0	0
Intersegment revenues	0.5	0.3	1.1	1.0
Depreciation and amortization expense	0.2	0.1	0.5	0.5
Miscellaneous income	3.2	2.9	10.3	9.9
Interest expense	0.7	0.5	1.7	1.6
Provision for income taxes	1.0	0.9	2.9	2.4
Preferred stock dividend requirements	0	0	0	0
Net income (loss) attributed to common shareholder	1.7	1.3	6.0	6.1
Reconciling Eliminations
Segment reporting information
External revenues	0	0	0	0
Intersegment revenues	(4.6)	(4.7)	(9.4)	(8.5)
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 for income taxes	0	0	0	0
Preferred stock dividend requirements	0	0		0
Net income (loss) attributed to common shareholder	0	0	0	0
Utility Segments
Segment reporting information
External revenues	371.9	378.1	1,173.1	1,119.8
Intersegment revenues	4.1	4.4	8.3	7.5
Depreciation and amortization expense	27.5	24.1	78.7	71.9
Miscellaneous income	2.8	0.9	6.6	1.5
Interest expense	10.0	10.0	30.2	30.3
Provision for income taxes	21.8	13.5	61.8	45.7
Preferred stock dividend requirements	(0.7)	(0.7)	(2.3)	(2.3)
Net income (loss) attributed to common shareholder	35.3	41.2	101.5	101.1
Utility Segments | Electric Utility
Segment reporting information
External revenues	331.7	343.1	946.0	922.6
Intersegment revenues	0	0	0	0
Depreciation and amortization expense	23.6	20.3	66.9	60.5
Miscellaneous income	2.8	0.8	6.5	1.4
Interest expense	7.9	8.0	23.8	24.4
Provision for income taxes	23.8	17.3	52.9	38.8
Preferred stock dividend requirements	(0.6)	(0.6)	(1.9)	(1.9)
Net income (loss) attributed to common shareholder	38.3	43.9	87.1	86.5
Utility Segments | Natural Gas Utility
Segment reporting information
External revenues	40.2	35.0	227.1	197.2
Intersegment revenues	4.1	4.4	8.3	7.5
Depreciation and amortization expense	3.9	3.8	11.8	11.4
Miscellaneous income	0	0.1	0.1	0.1
Interest expense	2.1	2.0	6.4	5.9
Provision for income taxes	(2.0)	(3.8)	8.9	6.9
Preferred stock dividend requirements	(0.1)	(0.1)	(0.4)	(0.4)
Net income (loss) attributed to common shareholder	$ (3.0)	$ (2.7)	$ 14.4	$ 14.6