SEGMENTS OF BUSINESS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013 segment	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting [Abstract]
Number of reportable segments									3
Segment reporting information
External revenues	$ 406.2	$ 371.9	$ 367.8	$ 433.4	$ 379.4	$ 378.1	$ 337.5	$ 404.2	$ 1,579.3	$ 1,499.2	$ 1,563.1
Intersegment revenues									0	0	0
Depreciation and amortization expense									106.2	96.2	95.6
Miscellaneous income									23.5	15.7	13.2
Interest expense									43.7	42.5	49.5
Provision for income taxes									81.9	62.6	73.2
Preferred stock dividend requirements									(3.1)	(3.1)	(3.1)
Net income attributed to common shareholder	27.3	37.0	25.9	44.6	24.5	42.5	22.6	42.1	134.8	131.7	122.8
Total assets	3,961.3				3,521.9				3,961.3	3,521.9	3,427.5
Capital expenditures for long-lived assets									633.2	179.5	91.5
Reconciling Eliminations
Segment reporting information
External revenues									0	0	0
Intersegment revenues									(12.3)	(10.6)	(11.6)
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
Capital expenditures for long-lived assets									0	0	0
Utility operations
Segment reporting information
External revenues									1,579.3	1,499.2	1,563.1
Intersegment revenues									10.9	9.2	10.2
Depreciation and amortization expense									106.1	96.1	95.5
Miscellaneous income									10.1	2.7	0.5
Interest expense									41.5	40.3	47.1
Provision for income taxes									77.7	59.1	69.7
Preferred stock dividend requirements									(3.1)	(3.1)	(3.1)
Net income attributed to common shareholder									127.3	124.0	116.2
Total assets	3,875.6				3,415.7				3,875.6	3,415.7	3,322.5
Capital expenditures for long-lived assets									633.2	179.5	91.2
Utility operations | Electric Utility
Segment reporting information
External revenues									1,241.8	1,212.0	1,220.7
Intersegment revenues									0	0	0
Depreciation and amortization expense									90.5	81.1	80.8
Miscellaneous income									9.9	2.6	0.6
Interest expense									33.0	32.4	38.1
Provision for income taxes									61.6	44.6	55.5
Preferred stock dividend requirements									(2.5)	(2.5)	(2.5)
Net income attributed to common shareholder									102.3	99.1	93.9
Total assets	3,241.8				2,747.5				3,241.8	2,747.5	2,689.8
Capital expenditures for long-lived assets									595.5	149.4	68.5
Utility operations | Natural Gas Utility
Segment reporting information
External revenues									337.5	287.2	342.4
Intersegment revenues									10.9	9.2	10.2
Depreciation and amortization expense									15.6	15.0	14.7
Miscellaneous income									0.2	0.1	(0.1)
Interest expense									8.5	7.9	9.0
Provision for income taxes									16.1	14.5	14.2
Preferred stock dividend requirements									(0.6)	(0.6)	(0.6)
Net income attributed to common shareholder									25.0	24.9	22.3
Total assets	633.8				668.2				633.8	668.2	632.7
Capital expenditures for long-lived assets									37.7	30.1	22.7
Nonutility operations | Other
Segment reporting information
External revenues									0	0	0
Intersegment revenues									1.4	1.4	1.4
Depreciation and amortization expense									0.6	0.6	0.6
Miscellaneous income									13.4	13.0	12.7
Interest expense									2.2	2.2	2.4
Provision for income taxes									4.2	3.5	3.5
Preferred stock dividend requirements									0	0	0
Net income attributed to common shareholder									7.5	7.7	6.6
Total assets	85.7				106.2				85.7	106.2	105.0
Capital expenditures for long-lived assets									$ 0	$ 0	$ 0.3