CONDENSED CONSOLIDATED 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	$ 827.5	$ 769.5	$ 1,832.7	$ 1,640.5
Operating expenses
Cost of sales	276.4	235.6	680.6	524.8
Other operation and maintenance	213.1	214.3	422.4	418.9
Depreciation and amortization	113.3	106.2	224.0	210.7
Property and revenue taxes	24.7	24.8	50.1	50.5
Total operating expenses	627.5	580.9	1,377.1	1,204.9
Operating income	200.0	188.6	455.6	435.6
Other income, net	7.3	4.9	14.7	10.2
Interest expense	116.0	116.6	232.2	235.2
Other expense	(108.7)	(111.7)	(217.5)	(225.0)
Income before income taxes	91.3	76.9	238.1	210.6
Income tax expense	12.2	6.0	31.7	20.7
Net income	79.1	70.9	206.4	189.9
Preferred stock dividend requirements	0.3	0.3	0.6	0.6
Net income attributed to common shareholder	$ 78.8	$ 70.6	$ 205.8	$ 189.3