Shareholder's Equity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2025	Jun. 30, 2025	Mar. 31, 2025	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2025	Sep. 30, 2024
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)			75,300,000				75,300,000
Beginning balance	$ 2,929.1	$ 2,683.1	$ 2,682.3	$ 2,576.8	$ 2,680.6	$ 2,881.0	$ 2,682.3	$ 2,881.0
Beginning balance (in shares)			3,000,000.0				3,000,000.0
Net income	140.0	131.8	$ 89.8	136.0	111.3	115.3	$ 361.6	$ 362.6
Activity related to stock-based compensation	$ 11.8	12.9	7.1	$ 11.1	11.1	14.4
Shares purchased under share repurchase program (in shares)	0			170,771			552,593	1,409,786
Shares purchased under share repurchase program		(0.5)	(133.5)	$ (52.4)	(187.1)	(267.0)	$ (134.0)	$ (506.5)
Dividends declared	$ (15.8)	(15.0)	(15.2)		(14.5)	(14.6)
Other comprehensive income (loss), net of tax	$ (13.6)	116.8	52.6	80.6	(24.6)	(48.5)	$ 155.8	7.5
Ending balance (in shares)	75,300,000						75,300,000
Ending balance	$ 3,051.5	$ 2,929.1	$ 2,683.1	$ 2,752.1	$ 2,576.8	$ 2,680.6	$ 3,051.5	$ 2,752.1
Ending balance (in shares)	3,400,000						3,400,000
Dividends declared per share (in dollars per share)	$ 0.22	$ 0.21	$ 0.21		$ 0.20	$ 0.20
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning 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
Ending 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
Ending balance	$ 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	0.0	1.8	22.1	33.5	55.2	120.2	22.1	120.2
Activity related to stock-based compensation		(1.8)	(20.3)	(10.2)	(21.7)	(65.0)
Ending balance	0.0	0.0	1.8	23.3	33.5	55.2	0.0	23.3
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ (1,140.2)	$ (1,163.2)	$ (1,057.1)	$ (979.5)	$ (825.2)	$ (637.6)	$ (1,057.1)	$ (637.6)
Beginning balance (in shares)	3,400,000	3,500,000	3,000,000.0	2,700,000	2,300,000	1,800,000	3,000,000.0	1,800,000
Activity related to stock-based compensation	$ 12.8	$ 23.5	$ 27.4	$ 21.3	$ 32.8	$ 79.4
Activity related to stock -based compensation (in shares)		(100,000)	(100,000)	(100,000)	(100,000)	(200,000)
Shares purchased under share repurchase program (in shares)			600,000	200,000	500,000	700,000
Shares purchased under share repurchase program		$ (0.5)	$ (133.5)	$ (52.4)	$ (187.1)	$ (267.0)
Ending balance	$ (1,127.4)	$ (1,140.2)	$ (1,163.2)	$ (1,010.6)	$ (979.5)	$ (825.2)	$ (1,127.4)	$ (1,010.6)
Ending balance (in shares)	3,400,000	3,400,000	3,500,000	2,800,000	2,700,000	2,300,000	3,400,000	2,800,000
Retained earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 4,139.2	$ 4,031.2	$ 3,956.6	$ 3,720.9	$ 3,624.1	$ 3,523.4	$ 3,956.6	$ 3,523.4
Net income	140.0	131.8	89.8	136.0	111.3	115.3
Activity related to stock-based compensation	(1.0)	(8.8)
Dividends declared	(15.8)	(15.0)	(15.2)		(14.5)	(14.6)
Ending balance	4,262.4	4,139.2	4,031.2	3,856.9	3,720.9	3,624.1	4,262.4	3,856.9
Accumulated other comprehensive loss
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(88.7)	(205.5)	(258.1)	(216.9)	(192.3)	(143.8)	(258.1)	(143.8)
Other comprehensive income (loss), net of tax	(13.6)	116.8	52.6	80.6	(24.6)	(48.5)
Ending balance	$ (102.3)	$ (88.7)	$ (205.5)	$ (136.3)	$ (216.9)	$ (192.3)	$ (102.3)	$ (136.3)

WEST PHARMACEUTICAL SERVICES INC 10-Q