Earnings Per Share (Computation 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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net income	$ 111.7	$ 131.7	$ 100.0	$ 12.9	$ 114.0	$ 114.7	$ 110.7	$ 69.3	$ 356.3	$ 408.7	$ 359.0
Net loss from discontinued operations									0.8	0.1	1.3
Income from continuing operations									$ 357.1	$ 408.8	$ 360.3
Weighted-average shares outstanding - basic (in shares)									38.3	39.0	40.6
Add: Potential effect of diluted securities attributable to stock-based payments									0.3	0.4	0.5
Weighted-average shares outstanding - diluted (in shares)									38.6	39.4	41.1
Earnings per share – Basic:
Income from continuing operations (in usd per share)									$ 9.32	$ 10.49	$ 8.87
Loss from discontinued operations (in usd per share)									(0.02)	0	(0.03)
Net income (in usd per share)	$ 2.92	$ 3.44	$ 2.62	$ 0.34	$ 2.96	$ 2.97	$ 2.83	$ 1.75	9.30	10.49	8.84
Earnings per share – Diluted:
Income from continuing operations (in usd per share)									9.26	10.38	8.77
Loss from discontinued operations (in usd per share)									(0.02)	0	(0.03)
Net income (in usd per share)	$ 2.89	$ 3.42	$ 2.60	$ 0.33	$ 2.93	$ 2.94	$ 2.80	$ 1.73	$ 9.24	$ 10.38	$ 8.74