Common Equity Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Equity [Abstract]
Net income	$ 183.4	$ 183.6	$ 324.2	$ 280.3
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ 183.4	$ 183.6	$ 324.2	$ 280.3
Basic average common shares outstanding	199.7	199.7	199.7	199.7
Contingently issuable shares (performance and restricted stock units)	0.4	0.2	0.3	0.1
Diluted average common shares outstanding	200.1	199.9	200.0	199.8
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings per average common share	$ 0.92	$ 0.92	$ 1.62	$ 1.40
Diluted earnings per average common share	$ 0.92	$ 0.92	$ 1.62	$ 1.40
Anti-dilutive shares excluded from earnings per share calculation	0.0	0.0	0.0	0.0