Condensed Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Common Stock 2004 Buyback Plan [Member]	Common Stock	Surplus 2004 Buyback Plan [Member]	Surplus	Retained Earnings	Accumulated Other Comprehensive Income (Loss).	2004 Buyback Plan [Member]	Total
Balance at Dec. 31, 2014		$ 60,377		$ 701,764	$ 223,156	$ (377)		$ 984,920
Balance (in shares) at Dec. 31, 2014		24,150,702
Increase (Decrease) in Stockholders' Equity
Net income					48,798			48,798
Other Comprehensive Income, net of tax effects						(1,554)		(1,554)
Total comprehensive income								47,244
Cash dividends on Series A preferred stock					(11,363)			(11,363)
Stock options exercised		$ 76		863				939
Stock options exercised (in shares)		30,060
Employee stock purchases		$ 8		199				207
Employee stock purchases (in shares)		3,366
Restricted stock awards		$ 76		(76)
Restricted stock awards (in shares)		30,605
Common stock repurchased		$ (43)		(1,032)				(1,075)
Common stock repurchased (in shares)		(17,202)
Share-based compensation expense				2,907				2,907
Balance at Jun. 30, 2015		$ 60,494		704,625	260,591	(1,931)		$ 1,023,779
Balance (in shares) at Jun. 30, 2015		24,197,531						24,197,531
Balance (in shares) Preferred Stock at Jun. 30, 2015								0
Balance at Dec. 31, 2015		$ 60,407		703,929	298,919	(3,871)		$ 1,059,384
Balance (in shares) at Dec. 31, 2015		24,162,657						24,162,657
Balance (in shares) Preferred Stock at Dec. 31, 2015								0
Increase (Decrease) in Stockholders' Equity
Net income					49,010			$ 49,010
Other Comprehensive Income, net of tax effects						10,381		10,381
Total comprehensive income								59,391
Cash dividends on Series A preferred stock					(14,029)			(14,029)
Stock options exercised		$ 60		748				808
Stock options exercised (in shares)		24,073
Employee stock purchases		$ 9		218				227
Employee stock purchases (in shares)		3,729
Restricted stock awards		$ 99		(99)
Restricted stock awards (in shares)		39,556
Stock issued pursuant to restricted stock units.		$ 90		(90)
Stock issued pursuant to restricted stock units (in shares)		35,903
Common stock repurchased	$ (82)	$ (95)	$ (2,048)	(2,377)			$ (2,130)	(2,472)
Common stock repurchased (in shares)	(32,900)	(37,792)
Share-based compensation expense				3,164				3,164
Balance at Jun. 30, 2016		$ 60,488		$ 703,445	$ 333,900	$ 6,510		$ 1,104,343
Balance (in shares) at Jun. 30, 2016		24,195,226						24,195,226
Balance (in shares) Preferred Stock at Jun. 30, 2016								0