CONDENSED CONSOLIDATED INCOME STATEMENTS - USD ($) shares in Millions, $ 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	$ 1,676.2	$ 1,548.7	$ 4,367.6	$ 3,657.3
Operating expenses
Cost of sales	525.9	444.5	1,791.5	1,179.2
Other operation and maintenance	463.8	473.1	943.7	928.8
Depreciation and amortization	266.2	242.5	527.6	481.6
Property and revenue taxes	51.5	49.8	106.7	102.3
Total operating expenses	1,307.4	1,209.9	3,369.5	2,691.9
Operating income	368.8	338.8	998.1	965.4
Equity in earnings of transmission affiliates	41.3	52.9	83.9	92.7
Other income, net	39.7	28.6	72.5	34.2
Interest expense	120.0	124.4	239.5	253.8
Other expense	(39.0)	(42.9)	(83.1)	(126.9)
Income before income taxes	329.8	295.9	915.0	838.5
Income tax expense	54.1	53.8	129.0	143.8
Net income	275.7	242.1	786.0	694.7
Preferred stock dividends of subsidiary	0.3	0.3	0.6	0.6
Net (income) loss attributable to noncontrolling interests	(0.6)	0.2	(0.7)	0.0
Net income attributed to common shareholders	$ 276.0	$ 241.6	$ 786.1	$ 694.1
Earnings per share
Basic (in dollars per share)	$ 0.88	$ 0.77	$ 2.49	$ 2.20
Diluted (in dollars per share)	$ 0.87	$ 0.76	$ 2.49	$ 2.19
Weighted average common shares outstanding
Basic (in shares)	315.4	315.4	315.4	315.4
Diluted (in shares)	316.3	316.5	316.3	316.6