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] Cumulative Effect Adjustment [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income/(Loss) [Member] Cumulative Effect Adjustment [Member]	Accumulated Other Comprehensive Income/(Loss) [Member]	Treasury Stock [Member]	Deferred Compensation Arrangements [Member]	Cumulative Effect Adjustment [Member]	Total
Balance at Dec. 31, 2017		$ 51,264	$ 894,879		$ 700,557		$ (3,699)	$ (21,014)	$ 13,328		$ 1,635,315
Balance (in shares) at Dec. 31, 2017	50,696,077
Increase (Decrease) in Stockholders' Equity
Net income					168,641						168,641
Other comprehensive income/(loss), net of tax							(41,398)				(41,398)
Dividends declared:
Common					(73,843)						(73,843)
Common stock issued under employee stock plans		313	6,130								6,443
Common stock issued under employee stock plan (in shares)	312,998
Stock-based compensation			6,064								6,064
Distribution of stock under deferred compensation arrangements								1,898	(1,898)		0
Distribution of stock under deferred compensation arrangements (in shares)	35,233
Treasury stock purchased								(298)	298		0
Treasury stock purchased (in shares)	(5,142)
Treasury stock issued to benefit plan			4,675					7,886			12,561
Treasury stock issued to benefit plans, net (in shares)	218,658
Balance at Dec. 31, 2018		51,577	911,748	$ 208	795,563	$ (208)	(45,305)	(11,528)	11,728	$ 0	1,713,783
Balance (in shares) at Dec. 31, 2018	51,257,824
Increase (Decrease) in Stockholders' Equity
Net income					169,063						169,063
Other comprehensive income/(loss), net of tax							35,079				35,079
Dividends declared:
Common					(81,775)						(81,775)
Common stock issued under employee stock plans		398	6,517								6,915
Common stock issued under employee stock plan (in shares)	397,887
Stock-based compensation			5,285								5,285
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 purchased								(286)	286		0
Treasury stock purchased (in shares)	(4,576)
Treasury stock issued to benefit plan			2,723					4,161			6,884
Treasury stock issued to benefit plans, net (in shares)	110,357
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
Net income					164,676						164,676
Other comprehensive income/(loss), net of tax							72,303				72,303
Dividends declared:
Common					(88,484)						(88,484)
Common stock issued under employee stock plans		417	15,375								15,792
Common stock issued under employee stock plan (in shares)	416,614
Stock-based compensation			6,419								6,419
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 purchased								(271)	271		0
Treasury stock purchased (in shares)	(4,406)
Treasury stock issued to benefit plan			38					47			85
Treasury stock issued to benefit plans, net (in shares)	1,240
Balance at Dec. 31, 2020		$ 53,755	$ 1,025,163	$ 1,140	$ 960,183		$ 62,077	$ (6,198)	$ 9,127	$ 1,140	$ 2,104,107
Balance (in shares) at Dec. 31, 2020	53,593,127