Common Equity Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Equity [Abstract]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ 250.9	$ 100.2	$ 47.1	$ 205.1	$ 110.7	$ 55.0	$ 398.2	$ 370.8
Basic average common shares outstanding	200.2			199.7			200.1	199.7
Contingently issuable shares (performance and restricted stock units)	0.6			0.9			0.5	0.7
Diluted average common shares outstanding	200.8			200.6			200.6	200.4
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings per average common share	$ 1.25			$ 1.03			$ 1.99	$ 1.86
Diluted earnings per average common share	$ 1.25			$ 1.02			$ 1.98	$ 1.85
Anti-dilutive shares excluded from earnings per share calculation	0.0			0.0			0.0	0.0