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 Effect Adjustment	Cumulative Effect Adjustment Retained Earnings
Beginning balance (in shares) at Dec. 31, 2018		51,361
Beginning balance at Dec. 31, 2018	$ 864,267	$ 5,136	$ 778,501	$ 97,074	$ (3,384)	$ (13,060)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	64,288			45,958		18,330
Stock based compensation expense	4,028		4,028
Common stock transactions related to stock based employee benefit plans (in shares)		71
Common stock transactions related to stock based employee benefit plans	(2,772)	$ 7	(26)		(2,753)
Common stock issued for stock options (in shares)		29
Common stock issued for stock options	428	$ 3	425
Increase (decrease) in shares during period (in shares)		100
Increase (decrease) in stockholders' equity during period	65,972	$ 10	4,427	45,958	(2,753)	18,330
Ending balance (in shares) at Jun. 30, 2019		51,461
Ending balance at Jun. 30, 2019	930,239	$ 5,146	782,928	143,032	(6,137)	5,270
Beginning balance (in shares) at Mar. 31, 2019		51,414
Beginning balance at Mar. 31, 2019	896,424	$ 5,141	780,680	119,779	(4,959)	(4,217)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	32,740			23,253		9,487
Stock based compensation expense	1,899		1,899
Common stock transactions related to stock based employee benefit plans (in shares)		22
Common stock transactions related to stock based employee benefit plans	(1,188)	$ 2	(12)		(1,178)
Common stock issued for stock options (in shares)		25
Common stock issued for stock options	364	$ 3	361
Increase (decrease) in shares during period (in shares)		47
Increase (decrease) in stockholders' equity during period	33,815	$ 5	2,248	23,253	(1,178)	9,487
Ending balance (in shares) at Jun. 30, 2019		51,461
Ending balance at Jun. 30, 2019	930,239	$ 5,146	782,928	143,032	(6,137)	5,270
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