CONDENSED CONSOLIDATED STATEMENTS OF INCOME (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Income Statement [Abstract]
Operating revenues	$ 358.8	$ 367.8	$ 914.5	$ 801.2
Cost of fuel, natural gas, and purchased power	150.8	164.3	456.6	376.9
Operating and maintenance expense	130.6	116.9	252.7	224.2
Depreciation and amortization expense	28.3	27.8	55.9	51.2
Taxes other than income taxes	12.6	11.9	25.2	24.6
Operating income	36.5	46.9	124.1	124.3
Miscellaneous income	6.6	5.9	13.9	10.9
Interest expense	14.3	10.3	28.3	21.2
Other expense	(7.7)	(4.4)	(14.4)	(10.3)
Income before taxes	28.8	42.5	109.7	114.0
Provision for income taxes	10.9	15.8	40.7	41.9
Net income	17.9	26.7	69.0	72.1
Preferred stock dividend requirements	(0.8)	(0.8)	(1.6)	(1.6)
Net income attributed to common shareholder	$ 17.1	$ 25.9	$ 67.4	$ 70.5