SEGMENTS OF BUSINESS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014 segment	Dec. 31, 2013	Dec. 31, 2012
Segment reporting information
Number of reportable segments									3
Revenues	$ 397.4	$ 370.4	$ 358.8	$ 555.7	$ 406.2	$ 371.9	$ 367.8	$ 433.4	$ 1,682.3	$ 1,579.3	$ 1,499.2
Depreciation and amortization expense									113.7	106.2	96.2
Miscellaneous income									24.4	23.5	15.7
Interest expense									57.4	43.7	42.5
Provision for income taxes									84.7	81.9	62.6
Preferred stock dividend requirements									(3.1)	(3.1)	(3.1)
Net income attributed to common shareholder	28.0	42.2	17.1	50.3	27.3	37.0	25.9	44.6	137.6	134.8	131.7
Total assets	4,278.7				3,961.3				4,278.7	3,961.3	3,521.9
Cash expenditures for long-lived assets									329.1	633.2	179.5
Intersegment revenues
Segment reporting information
Revenues									0	0	0
Reconciling Eliminations
Segment reporting information
Revenues									0	0	0
Depreciation and amortization expense									(0.5)	(0.5)	(0.5)
Miscellaneous income									0	0	0
Interest expense									0	0	0
Provision for income taxes									0	0	0
Preferred stock dividend requirements									0	0	0
Net income attributed to common shareholder									0	0	0
Total assets	0				0				0	0	0
Cash expenditures for long-lived assets									0	0	0
Reconciling Eliminations | Intersegment revenues
Segment reporting information
Revenues									(13.8)	(12.3)	(10.6)
Utility operations
Segment reporting information
Revenues									1,682.3	1,579.3	1,499.2
Depreciation and amortization expense									113.6	106.1	96.1
Miscellaneous income									11.5	10.1	2.7
Interest expense									55.3	41.5	40.3
Provision for income taxes									80.7	77.7	59.1
Preferred stock dividend requirements									(3.1)	(3.1)	(3.1)
Net income attributed to common shareholder									130.4	127.3	124.0
Total assets	4,193.3				3,875.6				4,193.3	3,875.6	3,415.7
Cash expenditures for long-lived assets									329.1	633.2	179.5
Utility operations | Intersegment revenues
Segment reporting information
Revenues									12.4	10.9	9.2
Utility operations | Electric Utility
Segment reporting information
Revenues									1,222.4	1,241.8	1,212.0
Depreciation and amortization expense									97.4	90.5	81.1
Miscellaneous income									11.1	9.9	2.6
Interest expense									45.1	33.0	32.4
Provision for income taxes									64.1	61.6	44.6
Preferred stock dividend requirements									(2.6)	(2.5)	(2.5)
Net income attributed to common shareholder									104.7	102.3	99.1
Total assets	3,511.0				3,241.8				3,511.0	3,241.8	2,747.5
Cash expenditures for long-lived assets									279.3	595.5	149.4
Utility operations | Electric Utility | Intersegment revenues
Segment reporting information
Revenues									0	0	0
Utility operations | Natural Gas Utility
Segment reporting information
Revenues									459.9	337.5	287.2
Depreciation and amortization expense									16.2	15.6	15.0
Miscellaneous income									0.4	0.2	0.1
Interest expense									10.2	8.5	7.9
Provision for income taxes									16.6	16.1	14.5
Preferred stock dividend requirements									(0.5)	(0.6)	(0.6)
Net income attributed to common shareholder									25.7	25.0	24.9
Total assets	682.3				633.8				682.3	633.8	668.2
Cash expenditures for long-lived assets									49.8	37.7	30.1
Utility operations | Natural Gas Utility | Intersegment revenues
Segment reporting information
Revenues									12.4	10.9	9.2
Nonutility operations | Other
Segment reporting information
Revenues									0	0	0
Depreciation and amortization expense									0.6	0.6	0.6
Miscellaneous income									12.9	13.4	13.0
Interest expense									2.1	2.2	2.2
Provision for income taxes									4.0	4.2	3.5
Preferred stock dividend requirements									0	0	0
Net income attributed to common shareholder									7.2	7.5	7.7
Total assets	85.4				85.7				85.4	85.7	106.2
Cash expenditures for long-lived assets									0	0	0
Nonutility operations | Other | Intersegment revenues
Segment reporting information
Revenues									$ 1.4	$ 1.4	$ 1.4