Common Stock and Earnings Per Share - Reconciliation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share - Basic [Abstract]
Net Income	$ 49.7	$ 31.2	$ 34.2	$ 70.5	$ 61.1	$ 30.7	$ 31.3	$ 51.0	$ 41.4	$ 44.9	$ 36.9	$ 49.0	$ 185.6	$ 174.1	$ 172.2
Average Common Shares													51.6	51.3	50.8
Earnings Per Share	$ 0.96	$ 0.60	$ 0.66	$ 1.37	$ 1.19	$ 0.59	$ 0.61	$ 1.00	$ 0.81	$ 0.88	$ 0.73	$ 0.97	$ 3.59	$ 3.39	$ 3.39
Earnings Per Share - Diluted [Abstract]
Net Income (Loss) Available to Common Stockholders, Diluted													$ 185.6	$ 174.1	$ 172.2
Average Common Shares													51.7	51.5	51.0
Earnings Per Share	$ 0.96	$ 0.60	$ 0.66	$ 1.37	$ 1.18	$ 0.59	$ 0.61	$ 0.99	$ 0.81	$ 0.88	$ 0.72	$ 0.97	$ 3.59	$ 3.38	$ 3.38
Dilutive Securities (Shares)													0.1	0.2	0.2