Condensed Consolidated Income Statements (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income Statement [Abstract]
Operating revenues	$ 1,602.0	$ 991.2	$ 3,796.8	$ 2,379.1
Operating expenses
Cost of sales	508.3	353.0	1,347.2	966.9
Other operation and maintenance	522.0	337.0	1,053.5	617.7
Depreciation and amortization	190.0	103.5	377.9	206.1
Property and revenue taxes	49.6	31.9	96.8	63.8
Total operating expenses	1,269.9	825.4	2,875.4	1,854.5
Operating income	332.1	165.8	921.4	524.6
Equity in earnings of transmission affiliate	30.9	14.3	69.4	30.4
Other income, net	32.4	26.1	65.1	29.1
Interest expense	100.1	61.8	201.0	121.2
Other expense	(36.8)	(21.4)	(66.5)	(61.7)
Income before income taxes	295.3	144.4	854.9	462.9
Income tax expense	113.6	63.2	326.7	185.6
Net income	181.7	81.2	528.2	277.3
Preferred stock dividends of subsidiary	0.3	0.3	0.6	0.6
Net income attributed to common shareholders	$ 181.4	$ 80.9	$ 527.6	$ 276.7
Earnings per share
Basic (in dollars per share)	$ 0.57	$ 0.36	$ 1.67	$ 1.22
Diluted (in dollars per share)	$ 0.57	$ 0.35	$ 1.66	$ 1.21
Weighted average common shares outstanding
Basic (in shares)	315.6	227.5	315.6	226.5
Diluted (in shares)	317.0	229.1	317.0	228.2
Dividends per share of common stock (in dollars per share)	$ 0.4950	$ 0.8629	$ 0.9900	$ 1.2854