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, 2017	$ 502,507	$ 47,619	$ 140,960	$ 444,481	$ (124,909)	$ (5,644)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income available to common shareholders	59,283			59,283
Other comprehensive income (loss)	(14,976)					(14,976)
New Accounting Pronouncement	0					(2,657)
New Accounting Pronouncement | Accounting Standards Update 2016-01	(2,700)			2,657		(2,657)
Cash dividends declared	(22,404)			(22,404)
Stock-based compensation expense	1,697		1,697
Restricted awards granted	0		(1,494)		1,494
Exercise of stock options	1,149		(713)		1,862
Purchase of treasury shares	(15,230)				(15,230)
Ending balance at Sep. 30, 2018	512,026	47,619	140,450	484,017	(136,783)	(23,277)
Beginning Balance at Jun. 30, 2018	503,753	47,619	140,091	471,515	(136,520)	(18,952)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income available to common shareholders	20,692			20,692
Other comprehensive income (loss)	(4,325)					(4,325)
Cash dividends declared	(8,190)			(8,190)
Stock-based compensation expense	455		455
Exercise of stock options	181		(96)		277
Purchase of treasury shares	(540)				(540)
Ending balance at Sep. 30, 2018	512,026	47,619	140,450	484,017	(136,783)	(23,277)
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 available to common shareholders	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
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 available to common shareholders	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