Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Capital Surplus	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Income (Loss)
Beginning Balance at Dec. 31, 2018	$ 600,764	$ 47,619	$ 169,555	$ 485,967	$ (87,895)	$ (14,482)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	66,741			66,741
Other comprehensive income (loss)	25,877					25,877
Cash dividends declared	(26,775)			(26,775)
Stock-based compensation expense	1,901		1,901
Restricted awards granted	0		(1,557)		1,557
Exercise of stock options	526		(105)		631
Purchase of treasury shares	(19,431)				(19,431)
Ending balance at Sep. 30, 2019	$ 649,603	47,619	169,794	525,933	(105,138)	11,395
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared (in dollars per share)	$ 1.63
Exercise of stock options (in shares)	11,721
Purchase of treasury shares (in shares)	260,674
Beginning Balance at Jun. 30, 2019	$ 636,908	47,619	169,374	512,911	(98,084)	5,088
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	22,371			22,371
Other comprehensive income (loss)	6,307					6,307
Cash dividends declared	(9,349)			(9,349)
Stock-based compensation expense	527		527
Exercise of stock options	161		(107)		268
Purchase of treasury shares	(7,322)				(7,322)
Ending balance at Sep. 30, 2019	$ 649,603	47,619	169,794	525,933	(105,138)	11,395
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared (in dollars per share)	$ 0.57
Exercise of stock options (in shares)	3,581
Purchase of treasury shares (in shares)	98,724
Beginning Balance at Dec. 31, 2019	$ 657,983	47,619	170,309	539,253	(105,038)	5,840
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Impact of adopting CECL	(2,335)			(2,335)
Net income	67,374			67,374
Other comprehensive income (loss)	24,650					24,650
Cash dividends declared	(27,391)			(27,391)
Stock-based compensation expense	2,455		2,455
Restricted awards granted	0		(2,126)		2,126
Exercise of stock options	223		(112)		335
Purchase of treasury shares	(31,600)				(31,600)
Ending balance at Sep. 30, 2020	$ 691,359	47,619	170,526	576,901	(134,177)	30,490
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared (in dollars per share)	$ 1.71
Exercise of stock options (in shares)	4,921
Purchase of treasury shares (in shares)	491,998
Beginning Balance at Jun. 30, 2020	$ 693,750	47,619	169,881	565,804	(120,583)	31,029
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	20,126			20,126
Other comprehensive income (loss)	(539)					(539)
Cash dividends declared	(9,029)			(9,029)
Stock-based compensation expense	753		753
Restricted awards granted	0		(93)		93
Exercise of stock options	34		(15)		49
Purchase of treasury shares	(13,736)				(13,736)
Ending balance at Sep. 30, 2020	$ 691,359	$ 47,619	$ 170,526	$ 576,901	$ (134,177)	$ 30,490
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared (in dollars per share)	$ 0.57
Exercise of stock options (in shares)	755
Purchase of treasury shares (in shares)	230,861