CONDENSED CONSOLIDATED INCOME STATEMENTS - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Operating revenues	$ 1,548.7	$ 1,590.2	$ 3,657.3	$ 3,967.6
Operating expenses
Cost of sales	444.5	491.9	1,179.2	1,501.5
Other operation and maintenance	473.1	503.6	928.8	1,054.2
Depreciation and amortization	242.5	229.9	481.6	456.3
Property and revenue taxes	49.8	50.2	102.3	98.2
Total operating expenses	1,209.9	1,275.6	2,691.9	3,110.2
Operating income	338.8	314.6	965.4	857.4
Equity in earnings of transmission affiliates	52.9	36.9	92.7	73.0
Other income, net	28.6	23.6	34.2	54.5
Interest expense	124.4	124.1	253.8	248.5
Other expense	(42.9)	(63.6)	(126.9)	(121.0)
Income before income taxes	295.9	251.0	838.5	736.4
Income tax expense	53.8	15.2	143.8	80.2
Net income	242.1	235.8	694.7	656.2
Preferred stock dividends of subsidiary	0.3	0.3	0.6	0.6
Net (income) loss attributed to noncontrolling interests	(0.2)	0.2	0.0	0.2
Net income attributed to common shareholders	$ 241.6	$ 235.7	$ 694.1	$ 655.8
Earnings per share
Basic (in dollars per share)	$ 0.77	$ 0.75	$ 2.20	$ 2.08
Diluted (in dollars per share)	$ 0.76	$ 0.74	$ 2.19	$ 2.07
Weighted average common shares outstanding
Basic (in shares)	315.4	315.4	315.4	315.4
Diluted (in shares)	316.5	316.7	316.6	316.7