Shareholder's Equity - Change in Shareholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Accounting Standards Update [Extensible Enumeration]									Accounting Standards Update 2016-13 [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 2,032.6	$ 1,829.0	$ 1,854.5	$ 1,606.3	$ 1,490.2	$ 1,573.2	$ 1,854.5	$ 1,573.2
Net income	175.6	187.3	151.2	82.3	91.2	74.3	$ 514.1	247.8
Activity related to stock-based compensation	$ 16.6	21.8	2.5	13.9	20.4	12.8
Share purchased under share repurchase program (in shares)	0						479,000
Shares purchased under share repurchase program			(137.1)			(115.5)	$ (137.1)
Dividends declared		(12.5)	(12.5)	(11.9)		(11.8)
Other comprehensive (loss) income, net of tax	$ (22.5)	7.0	(29.6)	28.3	4.5	(42.7)	(45.1)	(9.9)
Ending balance	$ 2,202.3	$ 2,032.6	$ 1,829.0	$ 1,718.9	$ 1,606.3	1,490.2	$ 2,202.3	1,718.9	$ 1,573.2
Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance						$ (0.1)		$ (0.1)
Ending balance									$ (0.1)
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance (in shares)	75,300,000	75,300,000	75,300,000	75,300,000	75,300,000	75,300,000	75,300,000	75,300,000
Beginning balance	$ 18.8	$ 18.8	$ 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	75,300,000	75,300,000	75,300,000
Ending balance	$ 18.8	$ 18.8	$ 18.8	$ 18.8	$ 18.8	$ 18.8	$ 18.8	$ 18.8	$ 18.8
Capital in Excess of Par Value
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	248.0	246.7	267.3	263.1	267.6	272.7	267.3	272.7
Activity related to stock-based compensation	3.2	1.3	(20.6)	5.1	(4.5)	(5.1)
Ending balance	$ 251.2	$ 248.0	$ 246.7	$ 268.2	$ 263.1	$ 267.6	$ 251.2	$ 268.2	$ 272.7
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance (in shares)	1,300,000	1,500,000	1,300,000	1,500,000	1,700,000	1,200,000	1,300,000	1,200,000
Beginning balance	$ (261.2)	$ (281.7)	$ (167.7)	$ (190.8)	$ (215.7)	$ (118.1)	$ (167.7)	$ (118.1)
Activity related to stock -based compensation (in shares)	(100,000)	(200,000)	(300,000)	(100,000)	(200,000)	(300,000)
Activity related to stock-based compensation	$ 13.4	$ 20.5	$ 23.1	$ 8.8	$ 24.9	$ 17.9
Share purchased under share repurchase program (in shares)			500,000			800,000
Shares purchased under share repurchase program			$ (137.1)			$ (115.5)
Balance (in shares)	1,200,000	1,300,000	1,500,000	1,400,000	1,500,000	1,700,000	1,200,000	1,400,000	1,200,000
Ending balance	$ (247.8)	$ (261.2)	$ (281.7)	$ (182.0)	$ (190.8)	$ (215.7)	$ (247.8)	$ (182.0)	$ (118.1)
Retained earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	2,160.2	1,985.4	1,846.7	1,703.0	1,611.8	1,549.4	1,846.7	1,549.4
Net income	175.6	187.3	151.2	82.3	91.2	74.3
Dividends declared		(12.5)	(12.5)	(11.9)		(11.8)
Ending balance	2,335.8	2,160.2	1,985.4	1,773.4	1,703.0	1,611.8	2,335.8	1,773.4	1,549.4
Retained earnings | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance						(0.1)		(0.1)
Ending balance									(0.1)
Accumulated other comprehensive loss
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(133.2)	(140.2)	(110.6)	(187.8)	(192.3)	(149.6)	(110.6)	(149.6)
Other comprehensive (loss) income, net of tax	(22.5)	7.0	(29.6)	28.3	4.5	(42.7)
Ending balance	$ (155.7)	$ (133.2)	$ (140.2)	$ (159.5)	$ (187.8)	$ (192.3)	$ (155.7)	$ (159.5)	$ (149.6)