SEGMENTS OF BUSINESS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 segment	Mar. 31, 2013
Segment Reporting [Abstract]
Number of reportable segments	3
Segment reporting information
External revenues	$ 555.7	$ 433.4
Intersegment revenues	0	0
Depreciation and amortization expense	27.6	23.4
Miscellaneous income	7.3	5.0
Interest expense	14.0	10.9
Provision for income taxes	29.8	26.1
Preferred stock dividend requirements	(0.8)	(0.8)
Net income attributed to common shareholder	50.3	44.6
Other
Segment reporting information
External revenues	0	0
Intersegment revenues	0.3	0.3
Depreciation and amortization expense	0.2	0.1
Miscellaneous income	3.8	3.3
Interest expense	0.5	0.6
Provision for income taxes	1.0	0.9
Preferred stock dividend requirements	0	0
Net income attributed to common shareholder	2.4	1.8
Reconciling Eliminations
Segment reporting information
External revenues	0	0
Intersegment revenues	(4.7)	(2.1)
Depreciation and amortization expense	(0.1)	(0.1)
Miscellaneous income	0	0
Interest expense	0	0
Provision for income taxes	0	0
Preferred stock dividend requirements	0	0
Net income attributed to common shareholder	0	0
Utility Segments
Segment reporting information
External revenues	555.7	433.4
Intersegment revenues	4.4	1.8
Depreciation and amortization expense	27.5	23.4
Miscellaneous income	3.5	1.7
Interest expense	13.5	10.3
Provision for income taxes	28.8	25.2
Preferred stock dividend requirements	(0.8)	(0.8)
Net income attributed to common shareholder	47.9	42.8
Utility Segments | Electric Utility
Segment reporting information
External revenues	321.4	307.9
Intersegment revenues	0	0
Depreciation and amortization expense	23.5	19.5
Miscellaneous income	3.5	1.6
Interest expense	10.9	8.2
Provision for income taxes	15.5	14.6
Preferred stock dividend requirements	(0.7)	(0.7)
Net income attributed to common shareholder	27.2	25.7
Utility Segments | Natural Gas Utility
Segment reporting information
External revenues	234.3	125.5
Intersegment revenues	4.4	1.8
Depreciation and amortization expense	4.0	3.9
Miscellaneous income	0	0.1
Interest expense	2.6	2.1
Provision for income taxes	13.3	10.6
Preferred stock dividend requirements	(0.1)	(0.1)
Net income attributed to common shareholder	$ 20.7	$ 17.1