Condensed Consolidated Statements of Changes in Equity - USD ($) shares in Thousands, $ in Millions	Total	Ordinary shares	Additional paid-in capital	Retained earnings	Accumulated other comprehensive loss
Dividends (in dollars per share)	$ 0.18
Beginning Balance at Dec. 31, 2018	$ 1,836.1	$ 1.7	$ 1,893.8	$ 169.2	$ (228.6)
Beginning Balance (in shares) at Dec. 31, 2018		171,400
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	51.3			51.3
Other comprehensive loss, net of tax	2.7				2.7
Dividends declared, $0.19 per share	(31.0)			(31.0)
Exercise of options, net of shares tendered for payment	9.1		9.1
Exercise of options, net of shares tendered for payment (in shares)		300
Issuance of restricted shares, net of cancellations (in shares)		200
Issuance of restricted shares, net of cancellations	0.0
Shares surrendered by employees to pay taxes	(3.2)		(3.2)
Share-based compensation	5.4		5.4
Ending Balance (in shares) at Mar. 31, 2019		171,900
Ending Balance at Mar. 31, 2019	1,870.4	$ 1.7	1,905.1	189.5	(225.9)
Beginning Balance at Dec. 31, 2018	1,836.1	$ 1.7	1,893.8	169.2	(228.6)
Beginning Balance (in shares) at Dec. 31, 2018		171,400
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	257.9
Ending Balance (in shares) at Sep. 30, 2019		168,100
Ending Balance at Sep. 30, 2019	$ 1,868.8	$ 1.7	1,765.9	335.3	(234.1)
Dividends (in dollars per share)	$ 0.18
Beginning Balance at Mar. 31, 2019	$ 1,870.4	$ 1.7	1,905.1	189.5	(225.9)
Beginning Balance (in shares) at Mar. 31, 2019		171,900
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	114.3			114.3
Other comprehensive loss, net of tax	(4.3)				(4.3)
Dividends declared, $0.19 per share	(30.4)			(30.4)
Share repurchase (in shares)		(4,000)
Share repurchases	(150.0)		(150.0)
Exercise of options, net of shares tendered for payment	2.2		2.2
Exercise of options, net of shares tendered for payment (in shares)		100
Issuance of restricted shares, net of cancellations (in shares)		100
Issuance of restricted shares, net of cancellations	0.0
Shares surrendered by employees to pay taxes (in shares)		(100)
Shares surrendered by employees to pay taxes	(0.9)		(0.9)
Share-based compensation	5.3		5.3
Ending Balance (in shares) at Jun. 30, 2019		168,000
Ending Balance at Jun. 30, 2019	$ 1,806.6	$ 1.7	1,761.7	273.4	(230.2)
Dividends (in dollars per share)	$ 0.18
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 92.3			92.3
Other comprehensive loss, net of tax	(3.9)				(3.9)
Dividends declared, $0.19 per share	(30.4)			(30.4)
Exercise of options, net of shares tendered for payment	0.0
Exercise of options, net of shares tendered for payment (in shares)		100
Shares surrendered by employees to pay taxes	(0.4)		(0.4)
Share-based compensation	4.6		4.6
Ending Balance (in shares) at Sep. 30, 2019		168,100
Ending Balance at Sep. 30, 2019	$ 1,868.8	$ 1.7	1,765.9	335.3	(234.1)
Dividends (in dollars per share)	$ 0.19
Beginning Balance at Dec. 31, 2019	$ 1,953.9	$ 1.7	1,777.7	401.0	(226.5)
Beginning Balance (in shares) at Dec. 31, 2019		168,300
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	72.7			72.7
Other comprehensive loss, net of tax	1.3				1.3
Dividends declared, $0.19 per share	(31.6)			(31.6)
Share repurchase (in shares)		(3,000)
Share repurchases	(115.2)		(115.2)
Exercise of options, net of shares tendered for payment	8.8		8.8
Exercise of options, net of shares tendered for payment (in shares)		300
Issuance of restricted shares, net of cancellations (in shares)		200
Issuance of restricted shares, net of cancellations	0.0
Shares surrendered by employees to pay taxes (in shares)		(100)
Shares surrendered by employees to pay taxes	(3.6)		(3.6)
Share-based compensation	6.2		6.2
Ending Balance (in shares) at Mar. 31, 2020		165,700
Ending Balance at Mar. 31, 2020	1,892.5	$ 1.7	1,673.9	442.1	(225.2)
Beginning Balance at Dec. 31, 2019	1,953.9	$ 1.7	1,777.7	401.0	(226.5)
Beginning Balance (in shares) at Dec. 31, 2019		168,300
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 255.6
Share repurchase (in shares)	(3,000)
Ending Balance (in shares) at Sep. 30, 2020		166,300
Ending Balance at Sep. 30, 2020	$ 2,049.8	$ 1.7	1,698.8	561.6	(212.3)
Dividends (in dollars per share)	$ 0.19
Beginning Balance at Mar. 31, 2020	$ 1,892.5	$ 1.7	1,673.9	442.1	(225.2)
Beginning Balance (in shares) at Mar. 31, 2020		165,700
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	72.1			72.1
Other comprehensive loss, net of tax	10.4				10.4
Dividends declared, $0.19 per share	(31.7)			(31.7)
Exercise of options, net of shares tendered for payment	2.8		2.8
Exercise of options, net of shares tendered for payment (in shares)		100
Issuance of restricted shares, net of cancellations (in shares)		100
Issuance of restricted shares, net of cancellations	0.0
Shares surrendered by employees to pay taxes (in shares)		0
Shares surrendered by employees to pay taxes	(0.6)		(0.6)
Share-based compensation	4.2		4.2
Ending Balance (in shares) at Jun. 30, 2020		165,900
Ending Balance at Jun. 30, 2020	$ 1,949.7	$ 1.7	1,680.3	482.5	(214.8)
Dividends (in dollars per share)	$ 0.19
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 110.8			110.8
Other comprehensive loss, net of tax	2.5				2.5
Dividends declared, $0.19 per share	(31.7)			(31.7)
Exercise of options, net of shares tendered for payment	14.1		14.1
Exercise of options, net of shares tendered for payment (in shares)		400
Shares surrendered by employees to pay taxes (in shares)		0
Shares surrendered by employees to pay taxes	(0.4)		(0.4)
Share-based compensation	4.8		4.8
Ending Balance (in shares) at Sep. 30, 2020		166,300
Ending Balance at Sep. 30, 2020	$ 2,049.8	$ 1.7	$ 1,698.8	$ 561.6	$ (212.3)