CONDENSED CONSOLIDATED INCOME STATEMENTS - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Operating revenues	$ 2,286.5	$ 2,304.5
Operating expenses
Cost of sales	972.1	941.1
Other operation and maintenance	511.9	504.5
Depreciation and amortization	208.6	194.6
Property and revenue taxes	48.8	49.6
Total operating expenses	1,741.4	1,689.8
Operating income	545.1	614.7
Equity in earnings of transmission affiliates	32.8	41.9
Other income, net	7.5	18.3
Interest expense	106.7	104.7
Other expense	(66.4)	(44.5)
Income before income taxes	478.7	570.2
Income tax expense	88.3	213.3
Net income	390.4	356.9
Preferred stock dividends of subsidiary	0.3	0.3
Net income attributed to common shareholders	$ 390.1	$ 356.6
Earnings per share
Basic (in dollars per share)	$ 1.24	$ 1.13
Diluted (in dollars per share)	$ 1.23	$ 1.12
Weighted average common shares outstanding
Basic (in shares)	315.5	315.6
Diluted (in shares)	316.9	317.2
Dividends per share of common stock (in dollars per share)	$ 0.5525	$ 0.5200