CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Cumulative Effect Adjustment Retained Earnings [Member]	Cumulative Effect Adjustment	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, 2018				$ 51,577	$ 911,748	$ 795,563	$ (45,305)	$ (11,528)	$ 11,728	$ 1,713,783
Balance (in shares) at Dec. 31, 2018			51,257,824
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income						126,179				126,179
Other comprehensive income (loss), net of tax							47,775			47,775
Cash dividends declared:
Common						(60,488)				(60,488)
Common stock activity under employee stock ownership plans				266	2,246					2,512
Common stock activity under employee stock ownership plans (in shares)			266,127
Stock-based compensation					3,998					3,998
Distribution of stock under deferred compensation arrangements					1,064			830	(1,894)	0
Distribution of stock under deferred compensation arrangements (in shares)			32,431
Treasury stock issued to benefit plans, net					2,626			3,823	213	6,662
Treasury stock issued to benefit plans, net (in shares)			103,672
Balance at Sep. 30, 2019				51,843	921,682	861,254	2,470	(6,875)	10,047	1,840,421
Balance (in shares) at Sep. 30, 2019			51,660,054
Balance at Jun. 30, 2019				51,806	917,855	843,288	(4,621)	(8,797)	9,979	1,809,510
Balance (in shares) at Jun. 30, 2019			51,570,608
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income						39,218				39,218
Other comprehensive income (loss), net of tax							7,091			7,091
Cash dividends declared:
Common						(21,252)				(21,252)
Common stock activity under employee stock ownership plans				37	1,254					1,291
Common stock activity under employee stock ownership plans (in shares)			37,542
Stock-based compensation					1,241					1,241
Treasury stock issued to benefit plans, net					1,332			1,922	68	3,322
Treasury stock issued to benefit plans, net (in shares)			51,904
Balance at Sep. 30, 2019				51,843	921,682	861,254	2,470	(6,875)	10,047	1,840,421
Balance (in shares) at Sep. 30, 2019			51,660,054
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
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 (Topic ASC 326) at Sep. 30, 2020	$ 1,140	$ 1,140
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 (Topic ASC 326) at Sep. 30, 2020	$ 1,140	$ 1,140
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