CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating revenues
Electric, non-affiliates	$ 667.5	$ 720.7	$ 704.7
Electric, affiliates	176.6	157.9	177.2
Natural gas	136.4	141.6	122.4
Other	0.4	1.3	1.2
Total operating revenues	980.9	1,021.5	1,005.5
Operating expenses
Electric fuel and purchased power, non-affiliates	11.4	13.1	16.2
Purchased power, affiliates	393.9	410.9	421.6
Cost of natural gas sold and transported	65.3	69.1	62.3
Operating and maintenance expenses	200.6	201.9	201.2
Conservation program expenses	12.4	12.3	12.6
Depreciation and amortization	137.5	126.1	111.2
Taxes (other than income taxes)	29.2	28.7	27.8
Total operating expenses	850.3	862.1	852.9
Operating income	130.6	159.4	152.6
Other Income and Expenses [Abstract]
Other expense, net	(1.1)	(3.1)	(3.5)
Allowance for funds used during construction — equity	3.2	9.2	6.7
Interest charges and financing costs
Interest charges — includes other financing costs of $1.3, $1.8, and $1.9, respectively	39.4	39.3	35.0
Allowance for funds used during construction — debt	(1.3)	(4.1)	(2.8)
Total interest charges and financing costs	38.1	35.2	32.2
Income before income taxes	94.6	130.3	123.6
Income taxes	15.7	32.3	44.2
Net income	$ 78.9	$ 98.0	$ 79.4