Condensed Consolidated Income Statements (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
Operating revenues	$ 1,712.5	$ 1,698.7	$ 5,509.3	$ 4,077.8
Operating expenses
Cost of sales	554.7	590.6	1,901.9	1,557.5
Other operation and maintenance	517.5	535.9	1,571.0	1,153.6
Depreciation and amortization	191.6	176.5	569.5	382.6
Property and revenue taxes	49.7	50.0	146.5	113.8
Total operating expenses	1,313.5	1,353.0	4,188.9	3,207.5
Operating income	399.0	345.7	1,320.4	870.3
Equity in earnings of transmission affiliate	38.3	40.0	107.7	70.4
Other income, net	7.5	11.1	72.6	40.2
Interest expense	99.1	103.2	300.1	224.4
Other expense	(53.3)	(52.1)	(119.8)	(113.8)
Income before income taxes	345.7	293.6	1,200.6	756.5
Income tax expense	128.4	110.5	455.1	296.1
Net income	217.3	183.1	745.5	460.4
Preferred stock dividends of subsidiary	0.3	0.6	0.9	1.2
Net income attributed to common shareholders	$ 217.0	$ 182.5	$ 744.6	$ 459.2
Earnings per share
Basic (in dollars per share)	$ 0.69	$ 0.58	$ 2.36	$ 1.79
Diluted (in dollars per share)	$ 0.68	$ 0.58	$ 2.35	$ 1.78
Weighted average common shares outstanding
Basic (in shares)	315.6	315.7	315.6	256.2
Diluted (in shares)	316.9	317.1	317.0	257.8
Dividends per share of common stock (in dollars per share)	$ 0.4950	$ 0.0000	$ 1.4850	$ 1.2854