STATEMENTS OF INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues [Abstract]
Operating revenue	$ 1,825.8	$ 1,933.2	$ 1,918.0
Operating expenses
Electric fuel and purchased power	875.4	1,043.5	1,055.3
Operating and maintenance expenses	285.3	282.7	285.4
Demand side management program expenses	16.6	17.7	15.5
Depreciation and amortization	229.9	209.6	193.9
Taxes (other than income taxes)	71.9	68.0	67.0
Total operating expenses	1,479.1	1,621.5	1,617.1
Operating income	346.7	311.7	300.9
Other income (expense), net	2.2	(3.0)	(1.8)
Allowance for funds used during construction — equity	26.8	19.1	9.3
Interest charges and financing costs
Interest charges — includes other financing costs of $3.4, $2.9 and $2.5, respectively	99.3	84.5	86.2
Allowance for funds used during construction — debt	(12.3)	(8.9)	(5.4)
Total interest charges and financing costs	87.0	75.6	80.8
Income before income taxes	288.7	252.2	227.6
Income taxes	25.6	38.9	68.4
Net income	$ 263.1	$ 213.3	$ 159.2