Consolidated Income Statements - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Operating revenues	$ 7,523.1	$ 7,679.5	$ 7,648.5
Operating expenses
Cost of sales	2,678.8	2,897.9	2,822.8
Other operation and maintenance	2,184.8	2,270.5	2,056.1
Depreciation and amortization	926.3	845.8	798.6
Property and revenue taxes	201.8	196.9	194.9
Total operating expenses	5,991.7	6,211.1	5,872.4
Operating income	1,531.4	1,468.4	1,776.1
Equity in earnings of transmission affiliates	127.6	136.7	154.3
Other income, net	102.2	70.3	73.7
Interest expense	501.5	445.1	415.7
Other expense	(271.7)	(238.1)	(187.7)
Income before income taxes	1,259.7	1,230.3	1,588.4
Income tax expense	125.0	169.8	383.5
Net income	1,134.7	1,060.5	1,204.9
Preferred stock dividends of subsidiary	1.2	1.2	1.2
Net loss attributed to noncontrolling interests	0.5	0.0	0.0
Net income attributed to common shareholders	$ 1,134.0	$ 1,059.3	$ 1,203.7
Earnings per share
Basic (in dollars per share)	$ 3.60	$ 3.36	$ 3.81
Diluted (in dollars per share)	$ 3.58	$ 3.34	$ 3.79
Weighted average common shares outstanding
Basic (in shares)	315.4	315.5	315.6
Diluted (in shares)	316.7	316.9	317.2