Consolidated Statements Of Changes in Shareholders Equity (USD $) In Thousands, unless otherwise specified	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]
Balance at beginning of period at Dec. 31, 2009	$ 576,985	$ 672	$ 317,081	$ 297,579	$ (38,347)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	160,950			160,950
Other comprehensive income (loss), net of tax	19,399				19,399
Stock options exercised	9,962	6	9,956
Common stock shares withheld to satisfy employee tax obligations	(6,601)	(3)	(6,598)
Net restricted stock forfeited and restricted stock units vested	0	(2)	2
Restricted stock units issued as per contractual employment arrangements	1,035		1,035
Amortization of share-based payment expense	20,189		20,381	(192)
Net tax benefit associated with the vesting or exercise of share-based compensation	2,555		2,555
Shares issued to directors	654		654
Cash dividends declared (2010: $0.20, 2011: $0.23, 2012: $0.52, per common share)	(13,620)			(13,620)
Balance at end of period at Dec. 31, 2010	771,508	673	345,066	444,717	(18,948)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	171,416			171,416
Other comprehensive income (loss), net of tax	(34,970)				(34,970)
Stock options exercised	2,566	1	2,565
Common stock shares withheld to satisfy employee tax obligations	(18,575)	(2)	(18,573)
Net restricted stock forfeited and restricted stock units vested	0	3	(3)
Restricted stock units issued as per contractual employment arrangements	149		149
Amortization of share-based payment expense	18,918		18,918
Net tax benefit associated with the vesting or exercise of share-based compensation	6,982		6,982
Net impact of Convertible Notes conversions and Convertible Note Hedge exercises	(374)		(374)
Shares issued to directors	898		898
Cash dividends declared (2010: $0.20, 2011: $0.23, 2012: $0.52, per common share)	(14,851)			(14,851)
Balance at end of period at Dec. 31, 2011	903,667	675	355,628	601,282	(53,918)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	108,292			108,292
Other comprehensive income (loss), net of tax	3,543				3,543
Stock options exercised	966		966
Common stock shares withheld to satisfy employee tax obligations	(11,379)	(1)	(11,378)
Net restricted stock forfeited and restricted stock units vested	0	3	(3)
Amortization of share-based payment expense	19,240		19,240
Net tax benefit associated with the vesting or exercise of share-based compensation	2,792		2,792
Shares issued to directors	928		928
Cash dividends declared (2010: $0.20, 2011: $0.23, 2012: $0.52, per common share)	(35,223)			(35,223)
Balance at end of period at Dec. 31, 2012	$ 992,826	$ 677	$ 368,173	$ 674,351	$ (50,375)