Condensed Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Preferred Stock	Common Stock 2004 Buyback Plan [Member]	Common Stock Other Than 2004 Buyback Plan [Member]	Common Stock	Surplus 2004 Buyback Plan [Member]	Surplus Other Than 2004 Buyback Plan [Member]	Surplus	Retained Earnings	Accumulated Other Comprehensive Income (Loss).	2004 Buyback Plan [Member]	Other Than 2004 Buyback Plan [Member]	Total
Balance at Dec. 31, 2013	$ 1			$ 60,260			$ 762,354	$ 168,577	$ (9,723)			$ 981,469
Balance (in shares) at Dec. 31, 2013				24,104,124
Balance (in shares) Preferred Stock at Dec. 31, 2013	65,000
Increase (Decrease) in Stockholders' Equity
Net income								75,437				75,437
Other Comprehensive Income, net of tax effects									9,346			9,346
Comprehensive income												84,783
Cash dividends on Series A preferred stock								(1,073)				(1,073)
Cash dividends declared								(19,785)				(19,785)
Stock options exercised				$ 57			652					709
Stock options exercised (in shares)				22,878
Employee stock purchases				$ 38			788					826
Employee stock purchases (in shares)				14,863
Restricted stock awards				$ 59			(59)
Restricted stock awards (in shares)				23,560
Repurchase of Series A preferred stock	$ (1)						(64,999)					(65,000)
Repurchase of Series A preferred stock (in shares)	(65,000)
Common stock repurchased				$ (37)			(880)					(917)
Common stock repurchased (in shares)				(14,723)
Share-based compensation expense							3,908					3,908
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								99,473				99,473
Other Comprehensive Income, net of tax effects									(3,494)			(3,494)
Comprehensive income												95,979
Cash dividends declared								(23,710)				(23,710)
Stock options exercised				$ 89			1,026					1,115
Stock options exercised (in shares)				35,360
Employee stock purchases				$ 34			874					908
Employee stock purchases (in shares)				13,536
Restricted stock awards				$ 110			(110)
Restricted stock awards (in shares)				44,105
Value of common stock repurchased buyback plan		$ (150)	$ (53)		$ (4,119)	$ (1,309)				$ (4,269)	$ (1,362)
Common stock repurchased buyback plan (in shares)		(60,000)	(21,046)
Share-based compensation expense							5,803					5,803
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								101,282				$ 101,282
Other Comprehensive Income, net of tax effects									(4,340)			(4,340)
Comprehensive income												96,942
Cash dividends declared								(29,285)				(29,285)
Stock options exercised				$ 158			2,046					2,204
Stock options exercised (in shares)				63,527
Employee stock purchases				$ 36			889					925
Employee stock purchases (in shares)				14,516
Restricted stock awards				$ 106			(106)
Restricted stock awards (in shares)				42,226
Stock issued pursuant to restricted stock units.				$ 90			(90)
Stock issued pursuant to restricted stock units (in shares)				35,903
Common stock repurchased		$ (82)	$ (139)		$ (2,048)	$ (3,712)				$ (2,130)	$ (3,851)
Common stock repurchased (in shares)		(32,900)	(55,537)
Excess tax benefits in connection with equity awards							4,178					4,178
Share-based compensation expense							6,221					6,221
Balance at Dec. 31, 2016				$ 60,576			$ 711,307	$ 370,916	$ (8,211)			$ 1,134,588
Balance (in shares) at Dec. 31, 2016				24,230,392								24,230,392
Balance (in shares) Preferred Stock at Dec. 31, 2016												0