Shareholders' Equity Change in Shareholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	$ 1,396.3	$ 1,279.9
Cumulative effect of new accounting principle in period of adoption		11.4
Net income	55.4	43.6
Activity related to stock-based compensation	$ 10.7	7.2
Share purchased under share repurchase program (in shares)	800,000
Shares purchased under share repurchase program	$ (83.1)	(47.9)
Dividends declared	(10.6)	(10.4)
Other comprehensive income, net of tax	0.8	21.5
Balance	1,369.5	1,305.3
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	$ 18.8	$ 18.8
Balance (in shares)	75,300,000	75,200,000
Shares issued under stock plans (in shares)	0	100,000
Activity related to stock-based compensation	$ 0.0	$ 0.0
Balance	$ 18.8	$ 18.8
Balance (in shares)	75,300,000	75,300,000
Additional Paid-in Capital [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	$ 282.0	$ 309.3
Activity related to stock-based compensation	(3.5)	(0.8)
Balance	278.5	308.5
Treasury Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	$ (103.7)	$ (109.1)
Balance (in shares)	1,200,000	1,300,000
Shares issued under stock plans (in shares)	(200,000)	(100,000)
Activity related to stock-based compensation	$ 14.2	$ 8.0
Share purchased under share repurchase program (in shares)	800,000	500,000
Shares purchased under share repurchase program	$ (83.1)	$ (47.9)
Balance	$ (172.6)	$ (149.0)
Balance (in shares)	1,800,000	1,700,000
Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	$ 1,353.4	$ 1,178.2
Cumulative effect of new accounting principle in period of adoption		11.4
Net income	55.4	43.6
Dividends declared	10.6	(10.4)
Balance	1,398.2	1,222.8
AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	(154.2)	(117.3)
Other comprehensive income, net of tax	0.8	21.5
Balance	$ (153.4)	$ (95.8)