Consolidated Income Statements - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Operating revenues	$ 7,679.5	$ 7,648.5	$ 7,472.3
Cost of sales	2,897.9	2,822.8	2,647.4
Other operation and maintenance	2,270.5	2,056.1	2,171.3
Depreciation and amortization	845.8	798.6	762.6
Property and revenue taxes	196.9	194.9	194.7
Total operating expenses	6,211.1	5,872.4	5,776.0
Operating income	1,468.4	1,776.1	1,696.3
Equity in earnings of transmission affiliates	136.7	154.3	146.5
Other income, net	70.3	73.7	66.6
Interest expense	445.1	415.7	402.7
Other expense	(238.1)	(187.7)	(189.6)
Income before income taxes	1,230.3	1,588.4	1,506.7
Income tax expense	169.8	383.5	566.5
Net income	1,060.5	1,204.9	940.2
Preferred stock dividends of subsidiary	1.2	1.2	1.2
Net income attributed to common shareholders	$ 1,059.3	$ 1,203.7	$ 939.0
Earnings Per Share (Basic)
Basic (in shares)	$ 3.36	$ 3.81	$ 2.98
Earnings Per Share (Diluted)
Diluted (in shares)	$ 3.34	$ 3.79	$ 2.96
Weighted Average Common Shares Outstanding (Basic)
Basic (in shares)	315.5	315.6	315.6
Weighted Average Common Shares Outstanding (Diluted)
Diluted (in shares)	316.9	317.2	316.9