CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Paid-in Capital	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Income (Loss)	Cumulative change in accounting principle upon adoption of new accounting pronouncement	Cumulative change in accounting principle upon adoption of new accounting pronouncement Retained Earnings
Beginning balance (in shares) at Dec. 31, 2019		51,514
Beginning balance at Dec. 31, 2019	$ 985,639	$ 5,151	$ 786,242	$ 195,813	$ (6,032)	$ 4,465	$ (16,876)	$ (16,876)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	38,618			25,789		12,829
Stock based compensation expense	3,523		3,523
Common stock transactions related to stock based employee benefit plans (in shares)		377
Common stock transactions related to stock based employee benefit plans	(2,014)	$ 38	(47)		(2,005)
Common stock issued for stock options (in shares)		57
Common stock issued for stock options	682	$ 6	676
Issuance of common stock, pursuant to acquisition (in shares)		1,043
Issuance of common stock, pursuant to acquisition	21,031	$ 104	20,927
Increase (decrease) in shares during period (in shares)		1,477
Increase (decrease) in stockholders' equity during period	44,964	$ 148	25,079	8,913	(2,005)	12,829
Ending balance (in shares) at Jun. 30, 2020		52,991
Ending balance at Jun. 30, 2020	1,030,603	$ 5,299	811,321	204,726	(8,037)	17,294
Beginning balance (in shares) at Mar. 31, 2020		52,709
Beginning balance at Mar. 31, 2020	991,787	$ 5,271	809,533	179,646	(7,422)	4,759
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	37,615			25,080		12,535
Stock based compensation expense	1,523		1,523
Common stock transactions related to stock based employee benefit plans (in shares)		262
Common stock transactions related to stock based employee benefit plans	(604)	$ 26	(15)		(615)
Common stock issued for stock options (in shares)		20
Common stock issued for stock options	282	$ 2	280
Increase (decrease) in shares during period (in shares)		282
Increase (decrease) in stockholders' equity during period	38,816	$ 28	1,788	25,080	(615)	12,535
Ending balance (in shares) at Jun. 30, 2020		52,991
Ending balance at Jun. 30, 2020	1,030,603	$ 5,299	811,321	204,726	(8,037)	17,294
Beginning balance (in shares) at Dec. 31, 2020		55,243
Beginning balance at Dec. 31, 2020	1,130,402	$ 5,524	856,092	256,701	(8,285)	20,370
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	54,560			65,129		(10,569)
Stock based compensation expense	4,262		4,262
Common stock transactions related to stock based employee benefit plans (in shares)		114
Common stock transactions related to stock based employee benefit plans	(1,901)	$ 12	(18)		(1,895)
Common stock issued for stock options (in shares)		79
Common stock issued for stock options	2,270	$ 8	2,262
Dividends on common stock	(7,246)			(7,246)
Increase (decrease) in shares during period (in shares)		193
Increase (decrease) in stockholders' equity during period	51,945	$ 20	6,506	57,883	(1,895)	(10,569)
Ending balance (in shares) at Jun. 30, 2021		55,436
Ending balance at Jun. 30, 2021	1,182,347	$ 5,544	862,598	314,584	(10,180)	9,801
Beginning balance (in shares) at Mar. 31, 2021		55,294
Beginning balance at Mar. 31, 2021	1,155,349	$ 5,529	858,688	290,420	(8,693)	9,405
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	31,806			31,410		396
Stock based compensation expense	2,503		2,503
Common stock transactions related to stock based employee benefit plans (in shares)		94
Common stock transactions related to stock based employee benefit plans	(1,495)	$ 10	(18)		(1,487)
Common stock issued for stock options (in shares)		48
Common stock issued for stock options	1,430	$ 5	1,425
Dividends on common stock	(7,246)			(7,246)
Increase (decrease) in shares during period (in shares)		142
Increase (decrease) in stockholders' equity during period	26,998	$ 15	3,910	24,164	(1,487)	396
Ending balance (in shares) at Jun. 30, 2021		55,436
Ending balance at Jun. 30, 2021	$ 1,182,347	$ 5,544	$ 862,598	$ 314,584	$ (10,180)	$ 9,801