CONDENSED CONSOLIDATED STATEMENTS OF INCOME (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Statement [Abstract]
Operating revenues	$ 370.4	$ 371.9	$ 1,284.9	$ 1,173.1
Cost of fuel, natural gas, and purchased power	138.8	151.3	595.4	528.2
Operating and maintenance expense	113.9	115.9	366.6	340.1
Depreciation and amortization expense	28.7	27.6	84.6	78.8
Taxes other than income taxes	11.1	11.9	36.3	36.5
Operating income	77.9	65.2	202.0	189.5
Miscellaneous income	5.6	6.0	19.5	16.9
Interest expense	14.6	10.7	42.9	31.9
Other expense	(9.0)	(4.7)	(23.4)	(15.0)
Income before taxes	68.9	60.5	178.6	174.5
Provision for income taxes	26.0	22.8	66.7	64.7
Net income	42.9	37.7	111.9	109.8
Preferred stock dividend requirements	(0.7)	(0.7)	(2.3)	(2.3)
Net income attributed to common shareholder	$ 42.2	$ 37.0	$ 109.6	$ 107.5