CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Electric, Non-affiliate	$ 184.3	$ 200.4	$ 519.6	$ 550.2
Electric, affiliates	44.7	39.9	132.3	116.0
Natural gas	14.6	15.4	96.6	93.8
Other	0.1	0.3	0.3	0.9
Total operating revenues	243.7	256.0	748.8	760.9
Operating expenses
Electric Fuel and Purchased Power, Non-Affiliate	3.2	3.2	8.2	9.7
Related Party Transaction, Utilities Operating Expense, Purchased Power	94.9	106.8	298.3	309.7
Cost of Goods Sold	5.4	6.2	45.9	43.3
Operating and maintenance expenses	47.5	50.0	150.8	149.3
Conservation expenses	3.3	3.0	9.3	9.2
Depreciation and amortization	34.4	31.7	102.9	93.7
Taxes (other than income taxes)	7.3	7.1	22.0	21.5
Total operating expenses	196.0	208.0	637.4	636.4
Operating income	47.7	48.0	111.4	124.5
Other expense, net	(0.3)	(0.8)	(0.9)	(1.6)
Allowance for funds used during construction — equity	0.7	2.6	2.2	6.7
Interest charges and financing costs
Interest charges — includes other financing costs of $0.3, $0.5, $1.0 and $1.5, respectively	9.7	10.1	29.5	29.5
Allowance for funds used during construction — debt	(0.3)	(1.2)	(0.9)	(3.0)
Total interest charges and financing costs	9.4	8.9	28.6	26.5
Income before income taxes	38.7	40.9	84.1	103.1
Income taxes	9.3	9.9	22.2	25.5
Net income	$ 29.4	$ 31.0	$ 61.9	$ 77.6