CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (USD $) In Thousands, unless otherwise specified	Total	Common Stock	Treasury Stock	Shares Held Under Employee Participation Plan	Additional Paid-in Capital	Accumulated Other Comprehensive Income	Accumulated Earnings
Balance at Dec. 31, 2009	$ 613,825	$ 524	$ (2,068)	$ (1,150)	$ 475,805	$ 26,829	$ 113,885
Balance (in shares) at Dec. 31, 2009		52,462
Increase (Decrease) in Stockholders' Equity
Net income	130,515						130,515
Other comprehensive income	23,930					23,930
Stock-based compensation	6,518				6,518
Stock-based compensation (in shares)		48
Issuance of restricted shares, net of shares remitted	(399)		(399)
Issuance of restricted shares, net of shares remitted (in shares)		(12)
Shares held under employee participation plan	373			373
Exercise of stock options	862	4			858
Exercise of stock options (in shares)		142
Issuance of common stock, net of issuance costs	1,015				1,015
Issuance of common stock, net of issuance costs (in shares)		32
Net tax benefit on exercise of stock-based awards	1,739				1,739
Employee stock purchase plan	2,449				2,449
Employee stock purchase plan (in shares)		100
Balance at Dec. 31, 2010	780,827	528	(2,467)	(777)	488,384	50,759	244,400
Balance (in shares) at Dec. 31, 2010		52,772
Increase (Decrease) in Stockholders' Equity
Net income	127,252						127,252
Treasury Stock, Retired, Cost Method, Amount			2,467		(2,467)
Other comprehensive income	(19,406)					(19,406)
Stock-based compensation	7,854				7,854
Stock-based compensation (in shares)		322
Issuance of restricted shares, net of shares remitted	(4,061)				(4,061)
Issuance of restricted shares, net of shares remitted (in shares)		(76)
Shares held under employee participation plan	308			308
Exercise of stock options	1,350	4			1,346
Exercise of stock options (in shares)		71
Net tax benefit on exercise of stock-based awards	3,347				3,347
Employee stock purchase plan	3,516				3,516
Employee stock purchase plan (in shares)		94
Balance at Dec. 31, 2011	900,987	532	0	(469)	497,919	31,353	371,652
Balance (in shares) at Dec. 31, 2011		53,183
Increase (Decrease) in Stockholders' Equity
Net income	129,674						129,674
Other comprehensive income	18,279					18,279
Stock-based compensation	7,494				7,494
Stock-based compensation (in shares)		168
Issuance of restricted shares, net of shares remitted	(2,912)				(2,912)
Issuance of restricted shares, net of shares remitted (in shares)		(48)
Exercise of stock options	288	3			285
Exercise of stock options (in shares)		47
Issuance of common stock, net of issuance costs	369,520	69			369,451
Issuance of common stock, net of issuance costs (in shares)		6,900
Net tax benefit on exercise of stock-based awards	2,546				2,546
Employee stock purchase plan	6,196				6,196
Employee stock purchase plan (in shares)		135
Balance at Dec. 31, 2012	$ 1,432,072	$ 604	$ 0	$ (469)	$ 880,979	$ 49,632	$ 501,326
Balance (in shares) at Dec. 31, 2012		60,385