Consolidated Statements of Shareholders' Equity (USD $) In Thousands, except Share data	Total	Common stock [Member]	Additional paid-in capital [Member]	Retained earnings [Member]
Balance at Dec. 31, 2011	$ 2,844,851	$ 1,272	$ 1,110,105	$ 1,733,474
Balance (in shares) at Dec. 31, 2011		127,180,000
Net income	585,746			585,746
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	9,553	1	9,552
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares		124,000
Net issuance of common stock upon exercise of stock options	54,876	19	54,857
Net issuance of common stock upon exercise of stock options, shares		1,860,000
Excess tax benefit of stock options exercised	38,572		38,572
Share based compensation	19,996		19,996
Share repurchases, including fees	(1,445,287)	(162)	(149,172)	(1,295,953)
Share repurchases, including fees, shares		(16,201,000)
Balance at Dec. 31, 2012	2,108,307	1,130	1,083,910	1,023,267
Balance (in shares) at Dec. 31, 2012		112,963,000
Net income	670,292			670,292
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	10,663		10,663
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares		113,000
Net issuance of common stock upon exercise of stock options	59,745	14	59,731
Net issuance of common stock upon exercise of stock options, shares		1,393,000
Excess tax benefit of stock options exercised	30,811		30,811
Share based compensation	19,531		19,531
Share repurchases, including fees	(933,028)	(85)	(85,717)	(847,226)
Share repurchases, including fees, shares	(8,529,000)	(8,529,000)
Balance at Dec. 31, 2013	1,966,321	1,059	1,118,929	846,333
Balance (in shares) at Dec. 31, 2013	105,939,766	105,940,000
Net income	778,182			778,182
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	11,181	1	11,180
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares		86,000
Net issuance of common stock upon exercise of stock options	59,594	13	59,581
Net issuance of common stock upon exercise of stock options, shares		1,320,000
Excess tax benefit of stock options exercised	49,150		49,150
Share based compensation	20,474		20,474
Share repurchases, including fees	(866,484)	(57)	(64,385)	(802,042)
Share repurchases, including fees, shares	(5,743,000)	(5,743,000)
Balance at Dec. 31, 2014	$ 2,018,418	$ 1,016	$ 1,194,929	$ 822,473
Balance (in shares) at Dec. 31, 2014	101,602,935	101,603,000