Common Equity Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Equity [Abstract]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ 252.5	$ 112.9	$ 52.7	$ 177.4	$ 85.9	$ (491.8)	$ 418.1	$ (228.5)
Basic average common shares outstanding	200.2			200.1			200.1	200.1
Contingently issuable shares (performance and restricted stock units)	0.2			0.3			0.2	0.0
Diluted average common shares outstanding	200.4			200.4			200.3	200.1
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings (loss) per average common share	$ 1.26			$ 0.89			$ 2.09	$ (1.14)
Diluted earnings (loss) per average common share	$ 1.26			$ 0.89			$ 2.09	$ (1.14)
Anti-dilutive shares excluded from earnings per share calculation	0.0			0.0			0.0	0.3