Consolidated Statements of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Shares at Par Value	Additional Paid-In Capital	Retained Earnings	Retirement Benefit Obligations	Interest Rate Swaps	Total
BALANCE (in shares)	220,929
BALANCE	$ 2,537,650	$ 2,209	$ 2,791,264	$ (136,460)	$ (119,363)		$ (119,363)
Share-based compensation expense	$ 2,878		2,878
Proceeds from employee share purchase plan (in shares)	140
Proceeds from employee share purchase plan	$ 3,328	1	3,327
Exercise of stock options (in shares)	675
Exercise of stock options	$ 7,019	7	7,012
Net share-settled stock options (in shares)	248
Net share-settled stock options		3	(3)
Tax withholding payments for net share-settled equity awards	$ (3,966)		(3,966)
Changes in retirement benefit obligations, net of income tax	657				657		657
Unrecognized gain on interest rate swaps, net of income tax	0
Net income	26,816			26,816
Changes in retirement benefit obligations, net of income tax	3,596
Net income	$ 187,825
BALANCE (in shares)	221,992
BALANCE	$ 2,574,382	2,220	2,800,512	(109,644)	(118,706)		(118,706)
Share-based compensation expense	$ 5,139		5,139
Proceeds from employee share purchase plan (in shares)	171
Proceeds from employee share purchase plan	$ 4,401	2	4,399
Exercise of stock options (in shares)	581
Exercise of stock options	$ 3,925	5	3,920
Net share-settled stock options (in shares)	777
Net share-settled stock options		8	(8)
Vested restricted stock units—net (in shares)	280
Vested restricted stock units—net		3	(3)
Tax withholding payments for net share-settled equity awards	$ (21,727)		(21,727)
Changes in retirement benefit obligations, net of income tax	1,880				1,880		1,880
Unrecognized gain on interest rate swaps, net of income tax	0
Net income	$ 65,458			65,458
BALANCE (in shares)	223,801
BALANCE	$ 2,633,458	2,238	2,792,232	(44,186)	(116,826)		(116,826)
Share-based compensation expense	$ 6,482		6,482
Proceeds from employee share purchase plan (in shares)	184
Proceeds from employee share purchase plan	$ 4,348	2	4,346
Exercise of stock options (in shares)	348
Exercise of stock options	$ 3,960	4	3,956
Net share-settled stock options (in shares)	126
Net share-settled stock options		1	(1)
Tax withholding payments for net share-settled equity awards	$ (2,119)		(2,119)
Changes in retirement benefit obligations, net of income tax	1,059				1,059		1,059
Unrecognized gain on interest rate swaps, net of income tax	2,445					$ 2,445	2,445
Net income	$ 95,551			95,551
BALANCE (in shares)	224,459
BALANCE	$ 2,745,184	2,245	2,804,896	51,365	(115,767)	2,445	(113,322)
BALANCE (in shares)	214,963
BALANCE	$ 2,751,363	2,150	2,721,454	123,514	(103,192)	7,437	(95,755)
Share-based compensation expense	$ 6,560		6,560
Proceeds from employee share purchase plan (in shares)	134
Proceeds from employee share purchase plan	$ 3,819	1	3,818
Exercise of stock options (in shares)	580
Exercise of stock options	$ 7,454	6	7,448
Net share-settled stock options (in shares)	10
Vested restricted stock units—net (in shares)	6
Performance restricted shares—net (in shares)	249
Performance restricted shares—net		2	(2)
Tax withholding payments for net share-settled equity awards	$ (269)		(269)
Changes in retirement benefit obligations, net of income tax	603				603		603
Unrecognized gain on interest rate swaps, net of income tax	9,040					9,040	9,040
Net income	67,317			67,317
Changes in retirement benefit obligations, net of income tax	26,269
Net income	$ 307,136
BALANCE (in shares)	215,942
BALANCE	$ 2,845,887	2,159	2,739,009	190,831	(102,589)	16,477	(86,112)
Share-based compensation expense	$ 10,606		10,606
Proceeds from employee share purchase plan (in shares)	203
Proceeds from employee share purchase plan	$ 6,297	2	6,295
Exercise of stock options (in shares)	529
Exercise of stock options	$ 8,151	6	8,145
Net share-settled stock options (in shares)	20
Vested restricted stock units—net (in shares)	288
Vested restricted stock units—net		3	(3)
Tax withholding payments for net share-settled equity awards	$ (5,377)		(5,377)
Changes in retirement benefit obligations, net of income tax	25,231				25,231		25,231
Unrecognized gain on interest rate swaps, net of income tax	3,644					3,644	3,644
Net income	$ 125,533			125,533
BALANCE (in shares)	216,982
BALANCE	$ 3,019,972	2,170	2,758,675	316,364	(77,358)	20,121	(57,237)
Share-based compensation expense	$ 3,059		3,059
Proceeds from employee share purchase plan (in shares)	174
Proceeds from employee share purchase plan	$ 4,802	2	4,800
Exercise of stock options (in shares)	182
Exercise of stock options	$ 2,723	2	2,721
Net share-settled stock options (in shares)	5
Vested restricted stock units—net (in shares)	1
Performance restricted shares—net (in shares)	(42)
Performance restricted shares—net		(1)	1
Tax withholding payments for net share-settled equity awards	$ (106)		(106)
Changes in retirement benefit obligations, net of income tax	435				435		435
Unrecognized gain on interest rate swaps, net of income tax	1,554					1,554	1,554
Net income	$ 114,286			114,286
BALANCE (in shares)	217,302
BALANCE	$ 3,146,725	$ 2,173	$ 2,769,150	$ 430,650	$ (76,923)	$ 21,675	$ (55,248)