CONDENSED CONSOLIDATED INCOME STATEMENTS - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Operating revenues	$ 1,590.2	$ 1,672.5	$ 3,967.6	$ 3,959.0
Operating expenses
Cost of sales	491.9	547.7	1,501.5	1,519.8
Other operation and maintenance	503.6	537.7	1,054.2	1,049.6
Depreciation and amortization	229.9	206.7	456.3	415.3
Property and revenue taxes	50.2	49.6	98.2	98.4
Total operating expenses	1,275.6	1,341.7	3,110.2	3,083.1
Operating income	314.6	330.8	857.4	875.9
Equity in earnings of transmission affiliates	36.9	28.7	73.0	61.5
Other income, net	23.6	31.4	54.5	38.9
Interest expense	124.1	108.5	248.5	215.2
Other expense	(63.6)	(48.4)	(121.0)	(114.8)
Income before income taxes	251.0	282.4	736.4	761.1
Income tax expense	15.2	51.1	80.2	139.4
Net income	235.8	231.3	656.2	621.7
Preferred stock dividends of subsidiary	0.3	0.3	0.6	0.6
Net loss attributed to noncontrolling interests	0.2	0.0	0.2	0.0
Net income attributed to common shareholders	$ 235.7	$ 231.0	$ 655.8	$ 621.1
Earnings per share
Basic (in dollars per share)	$ 0.75	$ 0.73	$ 2.08	$ 1.97
Diluted (in dollars per share)	$ 0.74	$ 0.73	$ 2.07	$ 1.96
Weighted average common shares outstanding
Basic (in shares)	315.4	315.5	315.4	315.5
Diluted (in shares)	316.7	316.9	316.7	316.9