SEGMENTS OF BUSINESS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 segment	Jun. 30, 2013
Segment Reporting [Abstract]
Number of reportable segments			3
Segment reporting information
Revenues	$ 358.8	$ 367.8	$ 914.5	$ 801.2
Depreciation and amortization expense	28.3	27.8	55.9	51.2
Miscellaneous income	6.6	5.9	13.9	10.9
Interest expense	14.3	10.3	28.3	21.2
Provision for income taxes	10.9	15.8	40.7	41.9
Preferred stock dividend requirements	(0.8)	(0.8)	(1.6)	(1.6)
Net income attributed to common shareholder	17.1	25.9	67.4	70.5
Intersegment revenues
Segment reporting information
Revenues	0	0	0	0
Reconciling Eliminations
Segment reporting information
Revenues	0	0	0	0
Depreciation and amortization expense	(0.2)	(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 attributed to common shareholder	0	0	0	0
Reconciling Eliminations | Intersegment revenues
Segment reporting information
Revenues	(3.1)	(2.7)	(7.8)	(4.8)
Utility Segments
Segment reporting information
Revenues	358.8	367.8	914.5	801.2
Depreciation and amortization expense	28.3	27.8	55.8	51.2
Miscellaneous income	2.9	2.1	6.4	3.8
Interest expense	13.8	9.9	27.3	20.2
Provision for income taxes	9.9	14.8	38.7	40.0
Preferred stock dividend requirements	(0.8)	(0.8)	(1.6)	(1.6)
Net income attributed to common shareholder	14.8	23.4	62.7	66.2
Utility Segments | Intersegment revenues
Segment reporting information
Revenues	2.7	2.4	7.1	4.2
Utility Segments | Electric Utility
Segment reporting information
Revenues	291.4	306.4	612.8	614.3
Depreciation and amortization expense	24.2	23.8	47.7	43.3
Miscellaneous income	2.8	2.1	6.3	3.7
Interest expense	11.2	7.7	22.1	15.9
Provision for income taxes	9.7	14.5	25.2	29.1
Preferred stock dividend requirements	(0.6)	(0.6)	(1.3)	(1.3)
Net income attributed to common shareholder	14.9	23.1	42.1	48.8
Utility Segments | Electric Utility | Intersegment revenues
Segment reporting information
Revenues	0	0	0	0
Utility Segments | Natural Gas Utility
Segment reporting information
Revenues	67.4	61.4	301.7	186.9
Depreciation and amortization expense	4.1	4.0	8.1	7.9
Miscellaneous income	0.1	0	0.1	0.1
Interest expense	2.6	2.2	5.2	4.3
Provision for income taxes	0.2	0.3	13.5	10.9
Preferred stock dividend requirements	(0.2)	(0.2)	(0.3)	(0.3)
Net income attributed to common shareholder	(0.1)	0.3	20.6	17.4
Utility Segments | Natural Gas Utility | Intersegment revenues
Segment reporting information
Revenues	2.7	2.4	7.1	4.2
Nonutility Segments | Other
Segment reporting information
Revenues	0	0	0	0
Depreciation and amortization expense	0.2	0.2	0.4	0.3
Miscellaneous income	3.7	3.8	7.5	7.1
Interest expense	0.5	0.4	1.0	1.0
Provision for income taxes	1.0	1.0	2.0	1.9
Preferred stock dividend requirements	0	0	0	0
Net income attributed to common shareholder	2.3	2.5	4.7	4.3
Nonutility Segments | Other | Intersegment revenues
Segment reporting information
Revenues	$ 0.4	$ 0.3	$ 0.7	$ 0.6