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 $4920 and $2096 for six months ended June 30, 2013 and 2012, respectively	24,089		4	24,085
Stock option activity, including tax benefit of $4920 and $2096 for six months ended June 30, 2013 and 2012, respectively (in shares)			380,576
Restricted stock activity, including tax benefit of $840 and $382 for six months ended June 30, 2013 and 2012, respectively	2,230			2,230
Restricted stock activity, including tax benefit of $840 and $382 for six months ended June 30, 2013 and 2012, respectively (in shares)			(4,076)
Change in fair value of corporate debt securities	85					85
Foreign currency translation adjustment	(167)					(167)
Net income	145,286				145,286
Balance at Jun. 30, 2012	2,143,154		760	1,411,670	728,488	2,236
Balance (in shares) at Jun. 30, 2012		12,275	76,589,626
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 $4920 and $2096 for six months ended June 30, 2013 and 2012, respectively	48,574		11	48,563
Stock option activity, including tax benefit of $4920 and $2096 for six months ended June 30, 2013 and 2012, respectively (in shares)			1,159,426
Restricted stock activity, including tax benefit of $840 and $382 for six months ended June 30, 2013 and 2012, respectively	1,692			1,692
Restricted stock activity, including tax benefit of $840 and $382 for six months ended June 30, 2013 and 2012, respectively (in shares)			248,763
Change in fair value of corporate debt securities	(1,394)					(1,394)
Foreign currency translation adjustment	(999)					(999)
Net income	53,091				53,091
Balance at Jun. 30, 2013	$ 2,329,618		$ 780	$ 1,479,945	$ 848,264	$ 629
Balance (in shares) at Jun. 30, 2013		12,050	78,854,790