CONSOLIDATED STATEMENTS OF INCOME (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Statement [Abstract]
Operating revenues	$ 1,579.3	$ 1,499.2	$ 1,563.1
Cost of fuel, natural gas, and purchased power	722.5	702.4	731.9
Operating and maintenance expense	462.5	429.1	452.4
Depreciation and amortization expense	106.2	96.2	95.6
Taxes other than income taxes	48.1	47.3	47.8
Operating income	240.0	224.2	235.4
Miscellaneous income	23.5	15.7	13.2
Interest expense	43.7	42.5	49.5
Other expense	(20.2)	(26.8)	(36.3)
Income before taxes	219.8	197.4	199.1
Provision for income taxes	81.9	62.6	73.2
Net income	137.9	134.8	125.9
Preferred stock dividend requirements	(3.1)	(3.1)	(3.1)
Net income attributed to common shareholder	$ 134.8	$ 131.7	$ 122.8