CONDENSED INCOME STATEMENTS - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Operating revenues	$ 381.1	$ 367.7	$ 1,127.6	$ 1,049.9
Operating expenses
Cost of sales	131.3	105.3	413.0	330.3
Other operation and maintenance	102.2	105.2	294.0	296.3
Depreciation and amortization	47.4	43.5	140.0	129.7
Property and revenue taxes	9.9	10.9	30.1	30.5
Total operating expenses	290.8	264.9	877.1	786.8
Operating income	90.3	102.8	250.5	263.1
Other income, net	8.1	8.7	27.1	25.4
Interest expense	16.1	15.6	48.6	47.9
Other expense	(8.0)	(6.9)	(21.5)	(22.5)
Income before income taxes	82.3	95.9	229.0	240.6
Income tax expense	10.4	19.6	27.6	48.4
Net income	$ 71.9	$ 76.3	$ 201.4	$ 192.2