SEGMENTS OF BUSINESS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 segment	Jun. 30, 2012
Segment Reporting [Abstract]
Number of reportable segments			3
Segment reporting information
External revenues	$ 367.8	$ 337.5	$ 801.2	$ 741.7
Intersegment revenues	0	0	0	0
Depreciation and amortization expense	27.8	24.0	51.2	47.9
Miscellaneous income	5.9	4.5	10.9	7.6
Interest expense	10.3	10.6	21.2	21.4
Provision for income taxes	15.8	12.3	41.9	33.7
Preferred stock dividend requirements	(0.8)	(0.8)	(1.6)	(1.6)
Net income (loss) attributed to common shareholder	25.9	22.6	70.5	64.7
Other
Segment reporting information
External revenues	0	0	0	0
Intersegment revenues	0.3	0.3	0.6	0.7
Depreciation and amortization expense	0.2	0.2	0.3	0.4
Miscellaneous income	3.8	4.0	7.1	7.0
Interest expense	0.4	0.6	1.0	1.1
Provision for income taxes	1.0	0.2	1.9	1.5
Preferred stock dividend requirements	0	0	0	0
Net income (loss) attributed to common shareholder	2.5	3.6	4.3	4.8
Reconciling Eliminations
Segment reporting information
External revenues	0	0	0	0
Intersegment revenues	(2.7)	(2.1)	(4.8)	(3.8)
Depreciation and amortization expense	(0.2)	(0.2)	(0.3)	(0.3)
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	0
Net income (loss) attributed to common shareholder	0	0	0	0
Utility Segments
Segment reporting information
External revenues	367.8	337.5	801.2	741.7
Intersegment revenues	2.4	1.8	4.2	3.1
Depreciation and amortization expense	27.8	24.0	51.2	47.8
Miscellaneous income	2.1	0.5	3.8	0.6
Interest expense	9.9	10.0	20.2	20.3
Provision for income taxes	14.8	12.1	40.0	32.2
Preferred stock dividend requirements	(0.8)	(0.8)	(1.6)	(1.6)
Net income (loss) attributed to common shareholder	23.4	19.0	66.2	59.9
Utility Segments | Electric Utility
Segment reporting information
External revenues	306.4	292.5	614.3	579.5
Intersegment revenues	0	0	0	0
Depreciation and amortization expense	23.8	20.2	43.3	40.2
Miscellaneous income	2.1	0.5	3.7	0.6
Interest expense	7.7	8.1	15.9	16.4
Provision for income taxes	14.5	12.1	29.1	21.5
Preferred stock dividend requirements	(0.6)	(0.6)	(1.3)	(1.3)
Net income (loss) attributed to common shareholder	23.1	19.7	48.8	42.6
Utility Segments | Natural Gas Utility
Segment reporting information
External revenues	61.4	45.0	186.9	162.2
Intersegment revenues	2.4	1.8	4.2	3.1
Depreciation and amortization expense	4.0	3.8	7.9	7.6
Miscellaneous income	0	0	0.1	0
Interest expense	2.2	1.9	4.3	3.9
Provision for income taxes	0.3	0	10.9	10.7
Preferred stock dividend requirements	(0.2)	(0.2)	(0.3)	(0.3)
Net income (loss) attributed to common shareholder	$ 0.3	$ (0.7)	$ 17.4	$ 17.3