Common Equity Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Equity [Abstract]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ 52.7	$ (491.8)
Basic average common shares outstanding	200.1	200.2
Contingently issuable shares (performance and restricted stock units)	0.0	0.0
Diluted average common shares outstanding	200.1	200.2
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings (loss) per average common share	$ 0.26	$ (2.46)
Diluted earnings (loss) per average common share	$ 0.26	$ (2.46)
Anti-dilutive shares excluded from earnings per share calculation	0.0	0.3