Consolidated Statements of Shareholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common stock	Additional paid-in capital	Accumulated Other Comprehensive Income (Loss)	Retained earnings
Balance at Dec. 31, 2011	$ 2,308,125	$ 3,617	$ 1,108,885		$ 1,195,623
Balance, shares at Dec. 31, 2011		361,743,376
Net income	739,385				739,385
Other Comprehensive Income (Loss), Net of Tax	0
Public offering of common stock	81,528	78	81,450
Public offering of common stock, shares		7,832,314
Stock-based compensation	30,202		30,202
Excess tax benefit on stock-based compensation	15,618
Stock options:
Exercised	60	2	58
Exercised, shares	173,000	173,000
Repurchased and canceled	(2,951)	0	(2,951)
Repurchased and canceled, shares		(65,968)
Restricted stock:
Issued	18	18	0
Issued, shares		1,832,056
Repurchased and canceled	(8,285)	(2)	(8,283)
Repurchased and canceled, shares		(225,042)
Forfeited	(1)	(1)
Forfeited, shares		(80,374)
Balance at Dec. 31, 2012	3,163,699	3,712	1,224,979		1,935,008
Balance, shares at Dec. 31, 2012		371,209,362
Net income	764,219				764,219
Other Comprehensive Income (Loss), Net of Tax	0
Stock-based compensation	39,886		39,886
Stock options:
Repurchased and canceled			0
Restricted stock:
Issued	5	5	0
Issued, shares		522,518
Repurchased and canceled	(14,690)	(3)	(14,687)
Repurchased and canceled, shares		(277,050)
Forfeited	(1)	(1)
Forfeited, shares		(137,512)
Balance at Dec. 31, 2013	3,953,118	3,713	1,250,178		2,699,227
Balance, shares at Dec. 31, 2013	371,317,318	371,317,318
Net income	977,341				977,341
Other Comprehensive Income (Loss), Net of Tax	(385)			(385)
Stock-based compensation	54,343		54,343
Restricted stock:
Issued	14	14	0
Issued, shares		1,424,764
Repurchased and canceled	(16,583)	(3)	(16,580)
Repurchased and canceled, shares		(283,434)
Forfeited	(4)	(4)
Forfeited, shares		(453,146)
Balance at Dec. 31, 2014	$ 4,967,844	$ 3,720	$ 1,287,941	$ (385)	$ 3,676,568
Balance, shares at Dec. 31, 2014	372,005,502	372,005,502