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				54,189	8,582	62,771
Cash dividends on Series A preferred stock at annual dividend rate of 9%				(1,073)		(1,073)
Cash dividends declared on common stock				(14,474)		(14,474)
Stock options exercised		$ 23	248			271
Stock options exercised (in shares)		9,122
Employee stock purchases		$ 18	395			413
Employee stock purchases (in shares)		7,058
Restricted stock awards		$ 54	(54)
Restricted stock awards (in shares)		21,560
Common stock repurchased		$ (17)	(387)			(404)
Common stock repurchased (in shares)		(6,644)
Share-based compensation expense			3,022			3,022
Balance at Sep. 30, 2014		$ 60,338	700,579	207,219	(1,141)	$ 966,995
Balance (in shares) at Sep. 30, 2014		24,135,220				24,135,220
Balance at Dec. 31, 2014		$ 60,377	701,764	223,156	(377)	$ 984,920
Balance (in shares) at Dec. 31, 2014		24,150,702				24,150,702
Increase (Decrease) in Stockholders' Equity
Comprehensive income				73,938	2,252	$ 76,190
Cash dividends declared on common stock				(17,413)		(17,413)
Stock options exercised		$ 77	881			958
Stock options exercised (in shares)		30,560
Employee stock purchases		$ 17	427			444
Employee stock purchases (in shares)		6,971
Restricted stock awards		$ 102	(102)
Restricted stock awards (in shares)		41,105
Repurchase of Series A preferred stock	$ (1)		(64,999)			(65,000)
Repurchase of Series A preferred stock (in shares)	(65,000)
Common stock repurchased		$ (44)	(1,057)			(1,101)
Common stock repurchased (in shares)		(17,545)
Share-based compensation expense			4,314			4,314
Balance at Sep. 30, 2015		$ 60,529	$ 706,227	$ 279,681	$ 1,875	$ 1,048,312
Balance (in shares) at Sep. 30, 2015		24,211,793				24,211,793