Consolidated Statements Of Changes In Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Capital Surplus	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Income (Loss)
Beginning Balance at Dec. 31, 2016	$ 442,438	$ 46,518	$ 112,873	$ 417,017	$ (126,958)	$ (7,012)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	44,646			44,646
Other comprehensive income	3,235					3,235
Cash dividends declared	(20,662)			(20,662)
Stock-based compensation expense	1,653		1,653
Restricted awards granted	0		(1,351)		1,351
Issuance of shares of common stock	28,408	1,101	27,307
Exercise of stock options	597		(101)		698
Ending balance at Sep. 30, 2017	500,315	47,619	140,381	441,001	(124,909)	(3,777)
Beginning Balance at Jun. 30, 2017	492,507	47,619	139,972	433,944	(124,943)	(4,085)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	13,932			13,932
Other comprehensive income	308					308
Cash dividends declared	(6,875)			(6,875)
Stock-based compensation expense	443		443
Restricted awards granted	0		(34)		34
Ending balance at Sep. 30, 2017	500,315	47,619	140,381	441,001	(124,909)	(3,777)
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	59,283			59,283
Other comprehensive income	(14,976)					(14,976)
Adoption of new accounting pronouncement (see Note B)	0					(2,657)
Adoption of new accounting pronouncement (see Note B) | 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	20,692			20,692
Other comprehensive income	(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)