Condensed Consolidated Statements of Changes in Shareholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total USD ($)	Preferred Stock	Common Stock USD ($)	Additional Paid-In Capital USD ($)	Retained Earnings USD ($)	Accumulated Other Comprehensive Income USD ($)
Balance at Dec. 31, 2011	$ 1,971,631		$ 756	$ 1,385,355	$ 583,202	$ 2,318
Balance (in shares) at Dec. 31, 2011		12,275	76,213,126
Increase (Decrease) in Shareholders' Equity
Stock option activity, including tax benefit of $6,965 and $2,534 for nine months ended September 30, 2013 and 2012, respectively	31,470		5	31,465
Stock option activity, including tax benefit of $6,965 and $2,534 for nine months ended September 30, 2013 and 2012, respectively (in shares)			452,636
Restricted stock activity, including tax benefit of $840 and $398 for nine months ended September 30, 2013 and 2012, respectively	3,349			3,349
Restricted stock activity, including tax benefit of $840 and $398 for nine months ended September 30, 2013 and 2012, respectively (in shares)			(4,076)
Change in fair value of corporate debt securities	134					134
Foreign currency translation adjustment	565					565
Net income	191,732				191,732
Balance at Sep. 30, 2012	2,198,881		761	1,420,169	774,934	3,017
Balance (in shares) at Sep. 30, 2012		12,275	76,661,686
Balance at Dec. 31, 2012	2,250,929		769	1,451,965	795,173	3,022
Balance (in shares) at Dec. 31, 2012		12,275	77,446,601
Increase (Decrease) in Shareholders' Equity
Repurchase of preferred stock	(22,275)			(22,275)
Repurchase of preferred stock (in shares)		(225)
Stock option activity, including tax benefit of $6,965 and $2,534 for nine months ended September 30, 2013 and 2012, respectively	68,070		16	68,054
Stock option activity, including tax benefit of $6,965 and $2,534 for nine months ended September 30, 2013 and 2012, respectively (in shares)			1,588,164
Restricted stock activity, including tax benefit of $840 and $398 for nine months ended September 30, 2013 and 2012, respectively	3,184			3,184
Restricted stock activity, including tax benefit of $840 and $398 for nine months ended September 30, 2013 and 2012, respectively (in shares)			249,763
Change in fair value of corporate debt securities	(1,394)					(1,394)
Foreign currency translation adjustment	(697)					(697)
Net income	94,408				94,408
Balance at Sep. 30, 2013	$ 2,392,225		$ 785	$ 1,500,928	$ 889,581	$ 931
Balance (in shares) at Sep. 30, 2013		12,050	79,284,528