CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock Shares Outstanding [Member]	Common Stock Amount Issued [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive (Loss) Income [Member]	Treasury Stock [Member]	Deferred Compensation Arrangements [Member]	Total
Balance at Dec. 31, 2019		$ 51,975	$ 927,337	$ 882,851	$ (10,226)	$ (6,823)	$ 10,120	$ 1,855,234
Balance (in shares) at Dec. 31, 2019	51,793,923
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income				118,191				118,191
Other comprehensive income (loss), net of tax					94,188			94,188
Cumulative effect of change in accounting Principle - Current Expected Credit Losses				1,140				1,140
Cash dividends declared:
Common				(65,909)				(65,909)
Common stock activity under employee stock ownership plans		360	13,781					14,141
Common stock activity under employee stock ownership plans (in shares)	359,934
Stock-based compensation			4,648					4,648
Stock issued for acquisition		1,363	75,579					76,942
Stock issued for acquisition (in shares)	1,363,259
Distribution of stock under deferred compensation arrangements			415			849	(1,264)	0
Distribution of stock under deferred compensation arrangements (in shares)	22,497
Treasury stock issued to benefit plans, net			38			(157)	204	85
Treasury stock issued to benefit plans, net (in shares)	(2,020)
Balance at Sep. 30, 2020		53,698	1,021,798	936,273	83,962	(6,131)	9,060	2,098,660
Balance (in shares) at Sep. 30, 2020	53,537,593
Balance at Jun. 30, 2020		53,672	1,019,291	916,002	90,031	(6,066)	8,995	2,081,925
Balance (in shares) at Jun. 30, 2020	53,514,216
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income				42,809				42,809
Other comprehensive income (loss), net of tax					(6,069)			(6,069)
Cash dividends declared:
Common				(22,538)				(22,538)
Common stock activity under employee stock ownership plans		26	1,092					1,118
Common stock activity under employee stock ownership plans (in shares)	24,584
Stock-based compensation			1,415					1,415
Treasury stock issued to benefit plans, net						(65)	65	0
Treasury stock issued to benefit plans, net (in shares)	(1,207)
Balance at Sep. 30, 2020		53,698	1,021,798	936,273	83,962	(6,131)	9,060	2,098,660
Balance (in shares) at Sep. 30, 2020	53,537,593
Balance at Dec. 31, 2020		53,755	1,025,163	960,183	62,077	(6,198)	9,127	2,104,107
Balance (in shares) at Dec. 31, 2020	53,593,127
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income				146,130				146,130
Other comprehensive income (loss), net of tax					(125,903)			(125,903)
Cash dividends declared:
Common				(68,377)				(68,377)
Common stock activity under employee stock ownership plans		317	8,824					9,141
Common stock activity under employee stock ownership plans (in shares)	317,390
Stock-based compensation			4,836					4,836
Distribution of stock under deferred compensation arrangements			323			694	(1,017)	0
Distribution of stock under deferred compensation arrangements (in shares)	18,089
Treasury stock issued to benefit plans, net						(189)	189	0
Treasury stock issued to benefit plans, net (in shares)	(2,619)
Balance at Sep. 30, 2021		54,072	1,039,146	1,037,936	(63,826)	(5,693)	8,299	2,069,934
Balance (in shares) at Sep. 30, 2021	53,925,987
Balance at Jun. 30, 2021		54,064	1,037,088	1,015,742	(48,400)	(5,632)	8,238	2,061,100
Balance (in shares) at Jun. 30, 2021	53,918,695
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income				45,336				45,336
Other comprehensive income (loss), net of tax					(15,426)			(15,426)
Cash dividends declared:
Common				(23,142)				(23,142)
Common stock activity under employee stock ownership plans		8	464					472
Common stock activity under employee stock ownership plans (in shares)	8,122
Stock-based compensation			1,594					1,594
Treasury stock issued to benefit plans, net						(61)	61	0
Treasury stock issued to benefit plans, net (in shares)	830
Balance at Sep. 30, 2021		$ 54,072	$ 1,039,146	$ 1,037,936	$ (63,826)	$ (5,693)	$ 8,299	$ 2,069,934
Balance (in shares) at Sep. 30, 2021	53,925,987