Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Deferred Benefits for Directors [Member]
Beginning Balance at Dec. 31, 2018	$ 1,978,827	$ 113,758	$ 1,166,701	$ 737,581	$ (274)	$ (37,871)	$ (1,068)
Beginning Balance, shares at Dec. 31, 2018		54,598,134
Net income	40,337			40,337
Other comprehensive income	19,773					19,773
Comprehensive income	60,110
Common dividends declared	(16,916)			(16,916)
Treasury shares acquired, shares		(7)
Stock options exercised	20		(25)		45
Stock options exercised, shares		1,000
Stock compensation expense	1,075		1,075
Deferred benefits for directors- net	23		10				13
Ending Balance at Mar. 31, 2019	2,023,139	$ 113,758	1,167,761	761,002	(229)	(18,098)	(1,055)
Ending Balance, shares at Mar. 31, 2019		54,599,127
Beginning Balance at Dec. 31, 2019	2,593,921	$ 141,827	1,636,966	824,694	(9,463)	1,201	(1,304)
Beginning Balance, shares at Dec. 31, 2019		67,824,428
Net income	23,396			23,396
Other comprehensive income	39,940					39,940
Comprehensive income	63,336
Common dividends declared	(21,435)			(21,435)
Adoption of accounting standard	(26,591)			(26,591)
Treasury shares acquired	(24,972)				(24,972)
Treasury shares acquired, shares		(786,012)
Stock options exercised	445		(276)		721
Stock options exercised, shares		19,739
Stock compensation expense	1,370		1,370
Deferred benefits for directors- net	(14)		62				(76)
Ending Balance at Mar. 31, 2020	$ 2,586,060	$ 141,827	$ 1,638,122	$ 800,064	$ (33,714)	$ 41,141	$ (1,380)
Ending Balance, shares at Mar. 31, 2020		67,058,155