Common Equity Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Equity [Abstract]
Net income	$ 183.6	$ 111.2	$ 280.3	$ 241.9
Basic average common shares outstanding	199.7	199.7	199.7	199.6
Contingently issuable shares (performance and restricted stock units)	0.2	0.0	0.1	0.0
Diluted average common shares outstanding	199.9	199.7	199.8	199.6
Earnings Per Share, Basic and Diluted [Abstract]
Basic Earnings Per Average Common Share	$ 0.92	$ 0.55	$ 1.40	$ 1.21
Diluted Earnings Per Average Common Share	$ 0.92	$ 0.55	$ 1.40	$ 1.21
Anti-dilutive shares excluded from earnings per share calculation	0.0	0.0	0.0	0.0