CONDENSED INCOME STATEMENTS - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement [Abstract]
Operating revenues	$ 341.2	$ 312.8	$ 746.5	$ 682.2
Operating expenses
Cost of sales	113.6	93.3	281.7	225.0
Other operation and maintenance	96.4	99.3	191.8	191.1
Depreciation and amortization	46.6	43.2	92.6	86.2
Property and revenue taxes	10.0	9.6	20.2	19.6
Total operating expenses	266.6	245.4	586.3	521.9
Operating income	74.6	67.4	160.2	160.3
Other income, net	10.4	8.4	19.0	16.7
Interest expense	16.3	16.0	32.5	32.3
Other expense	(5.9)	(7.6)	(13.5)	(15.6)
Income before income taxes	68.7	59.8	146.7	144.7
Income tax expense	8.5	12.1	17.2	28.8
Net income	$ 60.2	$ 47.7	$ 129.5	$ 115.9