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 $2,717 and $1,126 for three months ended March 31, 2013 and 2012, respectively	12,921		2	12,919
Stock option activity, including tax benefit of $2,717 and $1,126 for three months ended March 31, 2013 and 2012, respectively (in shares)			214,632
Restricted stock activity, including tax benefit of $749 and $353 for the three months ended March 31, 2013 and 2012, respectively	1,109			1,109
Restricted stock activity, including tax benefit of $749 and $353 for the three months ended March 31, 2013 and 2012, respectively (in shares)			(4,076)
Change in fair value of corporate debt securities	(70)					(70)
Foreign currency translation adjustment	288					288
Net income	78,619				78,619
Balance at Mar. 31, 2012	2,064,498		758	1,399,383	661,821	2,536
Balance (in shares) at Mar. 31, 2012		12,275	76,423,682
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 $2,717 and $1,126 for three months ended March 31, 2013 and 2012, respectively	24,737		5	24,732
Stock option activity, including tax benefit of $2,717 and $1,126 for three months ended March 31, 2013 and 2012, respectively (in shares)			540,567
Restricted stock activity, including tax benefit of $749 and $353 for the three months ended March 31, 2013 and 2012, respectively	442			442
Restricted stock activity, including tax benefit of $749 and $353 for the three months ended March 31, 2013 and 2012, respectively (in shares)			188,013
Change in fair value of corporate debt securities	(99)					(99)
Foreign currency translation adjustment	(391)					(391)
Net income	65,271				65,271
Balance at Mar. 31, 2013	$ 2,318,614		$ 774	$ 1,454,864	$ 860,444	$ 2,532
Balance (in shares) at Mar. 31, 2013		12,050	78,175,181