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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 43.1	$ 103.5	$ 115.5	$ 43.5	$ 40.4	$ 101.7	$ 110.7	$ 24.9	$ 305.7	$ 277.8	$ 186.6
Net loss from discontinued operations									1.4	0.8	0.6
Income from continuing operations									$ 307.1	$ 278.6	$ 187.2
Weighted-average shares outstanding - basic (in shares)									42.2	43.4	44.9
Add: Potential effect of diluted securities attributable to stock-based payments									0.6	0.6	0.7
Weighted-average shares outstanding - diluted (in shares)									42.8	44.0	45.6
Earnings per share – Basic:
Income from continuing operations (in usd per share)									$ 7.28	$ 6.41	$ 4.17
Loss from discontinued operations (in usd per share)									(0.03)	(0.02)	(0.01)
Net income (in usd per share)	$ 1.03	$ 2.47	$ 2.73	$ 1.02	$ 0.94	$ 2.35	$ 2.54	$ 0.57	7.25	6.39	4.16
Earnings per share – Diluted:
Income from continuing operations (in usd per share)									7.17	6.34	4.11
Loss from discontinued operations (in usd per share)									(0.03)	(0.02)	(0.02)
Net income (in usd per share)	$ 1.02	$ 2.44	$ 2.69	$ 1.00	$ 0.93	$ 2.33	$ 2.51	$ 0.56	$ 7.14	$ 6.32	$ 4.09