Earnings Per Share (Reconciliation of Basic and Diluted Earnings Per Share Computations) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings per common share:
Net income	$ 47.6	$ 69.2	$ 32.7	$ 5.3	$ 77.5	$ 45.0	$ 51.8	$ 39.3	$ 154.8	$ 213.6	$ 245.3
Weighted average common shares outstanding (in thousands) (in shares)									31,594	32,674	35,816
Earnings per common share (in dollars per share)	$ 1.56	$ 2.20	$ 1.02	$ 0.16	$ 2.40	$ 1.40	$ 1.59	$ 1.17	$ 4.90	$ 6.54	$ 6.85
Earnings per common share - assuming dilution:
Net income	$ 47.6	$ 69.2	$ 32.7	$ 5.3	$ 77.5	$ 45.0	$ 51.8	$ 39.3	$ 154.8	$ 213.6	$ 245.3
Weighted average common shares outstanding (in thousands) (in shares)									31,594	32,674	35,816
Common equivalent shares:
Share-based awards (in shares)									264	309	340
Weighted average common shares outstanding - assuming dilution (in thousands) (in shares)									31,858	32,983	36,156
Earnings per common share assuming dilution (in dollars per share)	$ 1.54	$ 2.18	$ 1.01	$ 0.16	$ 2.38	$ 1.38	$ 1.58	$ 1.16	$ 4.86	$ 6.48	$ 6.78