Common Equity Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Equity [Abstract]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ 47.1	$ 55.0
Basic average common shares outstanding	199.9	199.7
Contingently issuable shares (performance and restricted stock units)	0.6	0.5
Diluted average common shares outstanding	200.5	200.2
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings per average common share	$ 0.24	$ 0.28
Diluted earnings per average common share	$ 0.24	$ 0.27
Anti-dilutive shares excluded from earnings per share calculation	0.0	0.0