Change in Shareholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	$ 1,369.5	$ 1,396.3	$ 1,305.3	$ 1,279.9	$ 1,396.3	$ 1,279.9
Cumulative effect of new accounting principle in period of adoption				11.4
Net income	66.1	55.4	56.1	43.6	$ 121.5	99.7
Activity related to stock-based compensation	19.7	10.7	12.2	7.2
Share purchased under share repurchase program (in shares)					800,000
Shares purchased under share repurchase program		(83.1)	(22.9)	(47.9)	$ (83.1)
Dividends declared	(11.4)	(10.6)	(20.9)	(10.4)
Other comprehensive income, net of tax	1.2	0.8	(47.9)	21.5	2.0	(26.4)
Balance	1,445.1	1,369.5	1,281.9	1,305.3	1,445.1	1,281.9
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	$ 18.8	$ 18.8	$ 18.8	$ 18.8	$ 18.8	$ 18.8
Balance (in shares)	75,300,000	75,300,000	75,300,000	75,200,000	75,300,000	75,200,000
Shares issued under stock plans (in shares)	0	0	0	100,000
Activity related to stock-based compensation	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Balance	$ 18.8	$ 18.8	$ 18.8	$ 18.8	$ 18.8	$ 18.8
Balance (in shares)	75,300,000	75,300,000	75,300,000	75,300,000	75,300,000	75,300,000
Additional Paid-in Capital [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	$ 278.5	$ 282.0	$ 308.5	$ 309.3	$ 282.0	$ 309.3
Activity related to stock-based compensation	(5.1)	(3.5)	(3.8)	(0.8)
Balance	273.4	278.5	304.7	308.5	273.4	304.7
Treasury Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	$ (172.6)	$ (103.7)	$ (149.0)	$ (109.1)	$ (103.7)	$ (109.1)
Balance (in shares)	1,800,000	1,200,000	1,700,000	1,300,000	1,200,000	1,300,000
Shares issued under stock plans (in shares)	(200,000)	(200,000)	(200,000)	(100,000)
Activity related to stock-based compensation	$ 24.8	$ 14.2	$ 16.0	$ 8.0
Share purchased under share repurchase program (in shares)		800,000	300,000	500,000
Shares purchased under share repurchase program		$ (83.1)	$ (22.9)	$ (47.9)
Balance	$ (147.8)	$ (172.6)	$ (155.9)	$ (149.0)	$ (147.8)	$ (155.9)
Balance (in shares)	1,600,000	1,800,000	1,800,000	1,700,000	1,600,000	1,800,000
Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	$ 1,398.2	$ 1,353.4	$ 1,222.8	$ 1,178.2	$ 1,353.4	$ 1,178.2
Cumulative effect of new accounting principle in period of adoption				11.4
Net income	66.1	55.4	56.1	43.6
Dividends declared	11.4	10.6	(20.9)	(10.4)
Balance	1,452.9	1,398.2	1,258.0	1,222.8	1,452.9	1,258.0
AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	(153.4)	(154.2)	(95.8)	(117.3)	(154.2)	(117.3)
Other comprehensive income, net of tax	1.2	0.8	(47.9)	21.5
Balance	$ (152.2)	$ (153.4)	$ (143.7)	$ (95.8)	$ (152.2)	$ (143.7)