CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock Outstanding [Member]	Common Stock Amount Issued [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income/(Loss) [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				75,382				75,382
Other comprehensive income (loss), net of tax					100,257			100,257
Cumulative effect of change in accounting Principle - Current Expected Credit Losses				530				530
Dividends declared:
Common				(43,371)				(43,371)
Common stock activity under employee stock plans		334	12,689					13,023
Common stock activity under employee stock plans (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 (loss), net of tax					6,587			6,587
Dividends declared:
Common				(22,004)				(22,004)
Common stock activity under employee stock plans		119	6,505					6,624
Common stock activity under employee stock plans (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
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				100,794				100,794
Other comprehensive income (loss), net of tax					(110,477)			(110,477)
Dividends declared:
Common				(45,235)				(45,235)
Common stock activity under employee stock plans		309	8,360					8,669
Common stock activity under employee stock plans (in shares)	309,268
Stock-based compensation			3,242					3,242
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						(128)	128	0
Treasury stock issued to benefit plans, net (in shares)	(1,789)
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
Balance at Mar. 31, 2021		54,019	1,034,225	990,504	(109,957)	(5,572)	8,178	1,971,397
Balance (in shares) at Mar. 31, 2021	53,874,979
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income				47,944				47,944
Other comprehensive income (loss), net of tax					61,557			61,557
Dividends declared:
Common				(22,706)				(22,706)
Common stock activity under employee stock plans		45	1,295					1,340
Common stock activity under employee stock plans (in shares)	44,493
Stock-based compensation			1,568					1,568
Treasury stock issued to benefit plans, net						(60)	60	0
Treasury stock issued to benefit plans, net (in shares)	777
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