Common Equity Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Equity [Abstract]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ (491.8)	$ 47.1
Basic average common shares outstanding	200.2	199.9
Contingently issuable shares (performance and restricted stock units)	0.0	0.6
Diluted average common shares outstanding	200.2	200.5
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings (loss) per average common share	$ (2.46)	$ 0.24
Diluted earnings (loss) per average common share	$ (2.46)	$ 0.24
Anti-dilutive shares excluded from earnings per share calculation	0.0	0.0