Consolidated Income Statements - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Operating revenues	$ 7,241.7	$ 7,523.1	$ 7,679.5
Operating expenses
Cost of sales	2,319.5	2,678.8	2,897.9
Other operation and maintenance	2,032.2	2,184.8	2,270.5
Depreciation and amortization	975.9	926.3	845.8
Property and revenue taxes	208.0	201.8	196.9
Total operating expenses	5,535.6	5,991.7	6,211.1
Operating income	1,706.1	1,531.4	1,468.4
Equity in earnings of transmission affiliates	175.8	127.6	136.7
Other income, net	79.5	102.2	70.3
Interest expense	493.7	501.5	445.1
Loss on debt extinguishment	38.4	0.0	0.0
Other expense	(276.8)	(271.7)	(238.1)
Income before income taxes	1,429.3	1,259.7	1,230.3
Income tax expense	227.9	125.0	169.8
Net income	1,201.4	1,134.7	1,060.5
Preferred stock dividends of subsidiary	1.2	1.2	1.2
Net (income) loss attributed to noncontrolling interests	(0.3)	0.5	0.0
Net income attributed to common shareholders	$ 1,199.9	$ 1,134.0	$ 1,059.3
Earnings per share
Basic (in dollars per share)	$ 3.80	$ 3.60	$ 3.36
Diluted (in dollars per share)	$ 3.79	$ 3.58	$ 3.34
Weighted average common shares outstanding
Basic (in shares)	315.4	315.4	315.5
Diluted (in shares)	316.5	316.7	316.9