CONDENSED CONSOLIDATED STATEMENTS OF INCOME (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Income Statement [Abstract]
Operating revenues	$ 555.7	$ 433.4
Cost of fuel, natural gas, and purchased power	305.8	212.6
Operating and maintenance expense	122.1	107.3
Depreciation and amortization expense	27.6	23.4
Taxes other than income taxes	12.6	12.7
Operating income	87.6	77.4
Miscellaneous income	7.3	5.0
Interest expense	14.0	10.9
Other expense	(6.7)	(5.9)
Income before taxes	80.9	71.5
Provision for income taxes	29.8	26.1
Net income	51.1	45.4
Preferred stock dividend requirements	(0.8)	(0.8)
Net income attributed to common shareholder	$ 50.3	$ 44.6