CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) - 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	Cumulative Effect, Period of Adoption, Adjustment [Member] Retained Earnings [Member]	Cumulative Effect, Period of Adoption, Adjustment [Member]
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				86,961				86,961
Other comprehensive income, net of tax					40,684			40,684
Cash dividends declared:
Common				(39,236)				(39,236)
Common stock activity under employee stock ownership plan		229	992					1,221
Common stock activity under employee stock ownership plan (in shares)	228,585
Stock-based compensation			2,757					2,757
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			1,294			1,901	145	3,340
Treasury stock issued to benefit plans, net (in shares)	51,768
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
Balance at Mar. 31, 2019		51,728	913,917	817,933	(26,762)	(9,601)	9,913	1,757,128
Balance (in shares) at Mar. 31, 2019	51,471,371
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income				45,015				45,015
Other comprehensive income, net of tax					22,141			22,141
Cash dividends declared:
Common				(19,660)				(19,660)
Common stock activity under employee stock ownership plan		78	1,987					2,065
Common stock activity under employee stock ownership plan (in shares)	77,666
Stock-based compensation			1,366					1,366
Treasury stock issued to benefit plans, net			585			804	66	1,455
Treasury stock issued to benefit plans, net (in shares)	21,571
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
Balance at Dec. 31, 2019		51,975	927,337	882,851	(10,226)	(6,823)	10,120	1,855,234
Balance (ASU 2016-13 [Member]) at Dec. 31, 2019									$ 530	$ 530
Balance (in shares) at Dec. 31, 2019	51,793,923
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income				75,382				75,382
Other comprehensive income, net of tax					100,257			100,257
Cash dividends declared:
Common				(43,371)				(43,371)
Common stock activity under employee stock ownership plan		334	12,689					13,023
Common stock activity under employee stock ownership plan (in shares)	333,556
Stock-based compensation			3,233					3,233
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			(92)	139	85
Treasury stock issued to benefit plans, net (in shares)	981
Balance at Jun. 30, 2020		53,672	1,019,291	915,392	90,031	(6,066)	8,995	2,081,315
Balance (in shares) at Jun. 30, 2020	53,514,216
Balance at Mar. 31, 2020		52,190	935,924	902,148	83,444	(6,005)	8,930	1,976,631
Balance (in shares) at Mar. 31, 2020	52,031,092
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income				35,248				35,248
Other comprehensive income, net of tax					6,587			6,587
Cash dividends declared:
Common				(22,004)				(22,004)
Common stock activity under employee stock ownership plan		119	6,505					6,624
Common stock activity under employee stock ownership plan (in shares)	118,968
Stock-based compensation			1,281					1,281
Stock issued for acquisition		1,363	75,579					76,942
Stock issued for acquisition (in shares)	1,363,259
Treasury stock issued to benefit plans, net			2			(61)	65	6
Treasury stock issued to benefit plans, net (in shares)	897
Balance at Jun. 30, 2020		$ 53,672	$ 1,019,291	$ 915,392	$ 90,031	$ (6,066)	$ 8,995	$ 2,081,315
Balance (in shares) at Jun. 30, 2020	53,514,216