Consolidated Statements of Changes in Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, Adjusted Balance	Common Stock	Common Stock Cumulative Effect, Period of Adoption, Adjusted Balance	Capital Surplus	Capital Surplus Cumulative Effect, Period of Adoption, Adjusted Balance	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings Cumulative Effect, Period of Adoption, Adjusted Balance	Treasury Stock	Treasury Stock Cumulative Effect, Period of Adoption, Adjusted Balance	Accumulated Other Comprehensive Income	Accumulated Other Comprehensive Income Cumulative Effect, Period of Adoption, Adjusted Balance
Beginning Balance at Dec. 31, 2019	$ 657,983	$ (2,335)	$ 655,648	$ 47,619	$ 47,619	$ 170,309	$ 170,309	$ 539,253	$ (2,335)	$ 536,918	$ (105,038)	$ (105,038)	$ 5,840	$ 5,840
Ending balance at Mar. 31, 2020	$ 685,228			47,619		170,096		556,718			(116,665)		27,460
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Beginning Balance at Dec. 31, 2019	$ 657,983	$ (2,335)	$ 655,648	47,619	$ 47,619	170,309	$ 170,309	539,253	$ (2,335)	$ 536,918	(105,038)	$ (105,038)	5,840	$ 5,840
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	47,249							47,249
Other comprehensive income	25,189												25,189
Cash dividends declared	(18,363)							(18,363)
Stock-based compensation expense	1,702					1,702
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	0					(2,033)					2,033
Exercise of stock options	189					(97)					286
Purchase of treasury shares	(17,864)										(17,864)
Ending balance at Jun. 30, 2020	693,750			47,619		169,881		565,804			(120,583)		31,029
Beginning Balance at Mar. 31, 2020	685,228			47,619		170,096		556,718			(116,665)		27,460
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	18,251							18,251
Other comprehensive income	3,569												3,569
Cash dividends declared	(9,165)							(9,165)
Stock-based compensation expense	693					693
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	0					(879)					879
Exercise of stock options	67					(29)					96
Purchase of treasury shares	(4,893)										(4,893)
Ending balance at Jun. 30, 2020	693,750			47,619		169,881		565,804			(120,583)		31,029
Beginning Balance at Dec. 31, 2020	701,106			47,619		171,304		589,988			(139,038)		31,233
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	41,962							41,962
Other comprehensive income	(8,667)												(8,667)
Cash dividends declared	(18,397)							(18,397)
Stock-based compensation expense	1,778					1,778
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	0					(1,860)					1,860
Exercise of stock options	537					(1,548)					2,085
Purchase of treasury shares	(22,843)										(22,843)
Ending balance at Jun. 30, 2021	695,476			47,619		169,674		613,553			(157,936)		22,566
Beginning Balance at Mar. 31, 2021	691,685			47,619		170,526		600,396			(142,484)		15,628
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	22,148							22,148
Other comprehensive income	6,938												6,938
Cash dividends declared	(8,991)							(8,991)
Stock-based compensation expense	625					625
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	0					(1,396)					1,396
Exercise of stock options	152					(81)					233
Purchase of treasury shares	(17,081)										(17,081)
Ending balance at Jun. 30, 2021	$ 695,476			$ 47,619		$ 169,674		$ 613,553			$ (157,936)		$ 22,566