CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (USD $) In Thousands, except Share data	Total	Common Stock	Additional Paid-In Capital	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Income (Loss)
Balance at Dec. 31, 2009	$ 1,312,631	$ 897	$ 801,574	$ 502,201	$ (853)	$ 8,812
Balance, shares at Dec. 31, 2009		89,716,000			41,000
Stock-based compensation activity, including tax benefit	54,424	(19)	(53,552)		(853)
Stock-based compensation activity, including tax benefit, shares		1,883,000			(41,000)
Issuance of common stock under Employee Stock Purchase Plan	1,592		1,592
Issuance of common stock under Employee Stock Purchase Plan, shares		48,000
Net gains on interest rate swap, net of tax	532					532
Other comprehensive gain (loss)	7,618					7,618
Net income for the year	153,132			153,132
Balance at Dec. 31, 2010	1,529,929	916	856,718	655,333	0	16,962
Balance, shares at Dec. 31, 2010		91,647,000			0
Treasury shares acquired	(12,704)				(12,704)
Treasury shares acquired, shares	247,443				247,000
Stock-based compensation awards issued in Apache acquisition	3,170		3,170
Stock-based compensation activity, including tax benefit	56,322	10	43,608		12,704
Stock-based compensation activity, including tax benefit, shares		955,000			(247,000)
Issuance of common stock under Employee Stock Purchase Plan	2,167	1	2,166
Issuance of common stock under Employee Stock Purchase Plan, shares		50,000
Net gains on interest rate swap, net of tax	0
Other comprehensive gain (loss)	(5,086)					(5,086)
Net income for the year	180,675			180,675
Balance at Dec. 31, 2011	1,754,473	927	905,662	836,008	0	11,876
Balance, shares at Dec. 31, 2011		92,652,000			0
Treasury shares acquired	(95,477)				(95,477)
Treasury shares acquired, shares	1,500,000				1,500,000
Stock-based compensation activity, including tax benefit	78,591	5	20,791		57,795
Stock-based compensation activity, including tax benefit, shares		525,000			(939,000)
Issuance of common stock under Employee Stock Purchase Plan	2,446		915		1,531
Issuance of common stock under Employee Stock Purchase Plan, shares		25,000			(25,000)
Net gains on interest rate swap, net of tax	0
Other comprehensive gain (loss)	(3,225)					(3,225)
Net income for the year	203,483			203,483
Balance at Dec. 31, 2012	$ 1,940,291	$ 932	$ 927,368	$ 1,039,491	$ (36,151)	$ 8,651
Balance, shares at Dec. 31, 2012		93,202,000			536,000