SEGMENTS OF BUSINESS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 segment	Mar. 31, 2012
SEGMENTS OF BUSINESS
Number of reportable segments	3
Segment reporting information
External revenues	$ 433.4	$ 404.2
Depreciation and amortization expense	23.4	23.9
Miscellaneous income	5.0	3.1
Interest expense	10.9	10.8
Provision for income taxes	26.1	21.4
Preferred stock dividend requirements	(0.8)	(0.8)
Net income attributed to common shareholder	44.6	42.1
Other
Segment reporting information
Intersegment revenues	0.3	0.4
Depreciation and amortization expense	0.1	0.2
Miscellaneous income	3.3	3.0
Interest expense	0.6	0.5
Provision for income taxes	0.9	1.3
Net income attributed to common shareholder	1.8	1.2
Reconciling Eliminations
Segment reporting information
Intersegment revenues	(2.1)	(1.7)
Depreciation and amortization expense	(0.1)	(0.1)
Utility Segments
Segment reporting information
External revenues	433.4	404.2
Intersegment revenues	1.8	1.3
Depreciation and amortization expense	23.4	23.8
Miscellaneous income	1.7	0.1
Interest expense	10.3	10.3
Provision for income taxes	25.2	20.1
Preferred stock dividend requirements	(0.8)	(0.8)
Net income attributed to common shareholder	42.8	40.9
Utility Segments | Electric Utility
Segment reporting information
External revenues	307.9	287.0
Depreciation and amortization expense	19.5	20.0
Miscellaneous income	1.6	0.1
Interest expense	8.2	8.3
Provision for income taxes	14.6	9.4
Preferred stock dividend requirements	(0.7)	(0.7)
Net income attributed to common shareholder	25.7	22.9
Utility Segments | Natural Gas Utility
Segment reporting information
External revenues	125.5	117.2
Intersegment revenues	1.8	1.3
Depreciation and amortization expense	3.9	3.8
Miscellaneous income	0.1
Interest expense	2.1	2.0
Provision for income taxes	10.6	10.7
Preferred stock dividend requirements	(0.1)	(0.1)
Net income attributed to common shareholder	$ 17.1	$ 18.0