Condensed Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Preferred Stock	Corporate Stocks	Surplus	Retained Earnings	Accumulated Other Comprehensive Income (Loss).	Total
Balance at Dec. 31, 2013	$ 1	$ 60,260	$ 762,354	$ 168,577	$ (9,723)	$ 981,469
Balance (in shares) at Dec. 31, 2013	65,000	24,104,124
Increase (Decrease) in Stockholders' Equity
Comprehensive income				34,863	10,131	44,994
Cash dividends on Series A preferred stock at annual dividend rate of 9%				(1,073)		(1,073)
Cash dividends declared on common stock at $0.39 and $0.47 per share				(9,406)		(9,406)
Employee stock purchases		$ 8	185			193
Employee stock purchases (in shares)		3,251
Stock options exercised		$ 12	117			129
Stock options exercised (in shares)		4,660
Restricted stock awards		$ 57	(57)
Restricted stock awards (in shares)		22,810
Repurchase of Series A preferred stock	$ (1)		(64,999)			(65,000)
Repurchase of Series A preferred stock (in shares)	(65,000)
Common stock repurchased		$ (12)	(283)			(295)
Common stock repurchased (in shares)		(4,839)
Share-based compensation expense			2,007			2,007
Balance at Jun. 30, 2014		$ 60,325	699,324	192,961	408	953,018
Balance (in shares) at Jun. 30, 2014		24,130,006
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
Comprehensive income				48,798	(1,554)	47,244
Cash dividends declared on common stock at $0.39 and $0.47 per share				(11,363)		(11,363)
Employee stock purchases		$ 8	199			207
Employee stock purchases (in shares)		3,366
Stock options exercised		$ 76	863			939
Stock options exercised (in shares)		30,060
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