Consolidated Income Statements - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Operating revenues	$ 7,472.3	$ 5,926.1	$ 4,997.1
Cost of sales	2,647.4	2,240.1	2,259.4
Other operation and maintenance	2,185.5	1,709.3	1,112.4
Depreciation and amortization	762.6	561.8	391.4
Property and revenue taxes	194.7	164.4	121.8
Total operating expenses	5,790.2	4,675.6	3,885.0
Operating Income	1,682.1	1,250.5	1,112.1
Equity in earnings of transmission affiliate	146.5	96.1	66.0
Other income, net	80.8	58.9	13.4
Interest expense	402.7	331.4	240.3
Other expense	(175.4)	(176.4)	(160.9)
Income before income taxes	1,506.7	1,074.1	951.2
Income tax expense	566.5	433.8	361.7
Net income	940.2	640.3	589.5
Preferred stock dividends of subsidiary	1.2	1.8	1.2
Net income attributed to common shareholders	$ 939.0	$ 638.5	$ 588.3
Earnings Per Share (Basic)
Earnings per common share (basic) (in dollars per share)	$ 2.98	$ 2.36	$ 2.61
Earnings Per Share (Diluted)
Earnings per common share (diluted) (in dollars per share)	$ 2.96	$ 2.34	$ 2.59
Weighted Average Common Shares Outstanding (Basic)
Basic (in shares)	315.6	271.1	225.6
Weighted Average Common Shares Outstanding (Diluted)
Diluted (in shares)	316.9	272.7	227.5