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	94,934			71,563		23,371
Stock based compensation expense (in shares)		30
Stock based compensation expense	5,773		5,773
Common stock transactions related to stock based employee benefit plans (in shares)		62
Common stock transactions related to stock based employee benefit plans	(2,724)	$ 9	(38)		(2,695)
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)		121
Increase (decrease) in stockholders' equity during period	98,411	$ 12	6,160	71,563	(2,695)	23,371
Ending balance (in shares) at Sep. 30, 2019		51,482
Ending balance at Sep. 30, 2019	962,678	$ 5,148	784,661	168,637	(6,079)	10,311
Beginning balance (in shares) at Jun. 30, 2019		51,461
Beginning balance at Jun. 30, 2019	930,239	$ 5,146	782,928	143,032	(6,137)	5,270
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	30,646			25,605		5,041
Stock based compensation expense (in shares)		30
Stock based compensation expense	1,745		1,745
Common stock transactions related to stock based employee benefit plans (in shares)		(9)
Common stock transactions related to stock based employee benefit plans	48	$ 2	(12)		58
Increase (decrease) in shares during period (in shares)		21
Increase (decrease) in stockholders' equity during period	32,439	$ 2	1,733	25,605	58	5,041
Ending balance (in shares) at Sep. 30, 2019		51,482
Ending balance at Sep. 30, 2019	962,678	$ 5,148	784,661	168,637	(6,079)	10,311
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	63,175			48,417		14,758
Stock based compensation expense (in shares)		39
Stock based compensation expense	5,471		5,471
Common stock transactions related to stock based employee benefit plans (in shares)		395
Common stock transactions related to stock based employee benefit plans	(1,918)	$ 44	(53)		(1,909)
Common stock issued for stock options (in shares)		58
Common stock issued for stock options	698	$ 6	692
Issuance of common stock, pursuant to acquisition (in shares)		3,163
Issuance of common stock, pursuant to acquisition	62,152	$ 316	61,836
Increase (decrease) in shares during period (in shares)		3,655
Increase (decrease) in stockholders' equity during period	112,702	$ 366	67,946	31,541	(1,909)	14,758
Ending balance (in shares) at Sep. 30, 2020		55,169
Ending balance at Sep. 30, 2020	1,098,341	$ 5,517	854,188	227,354	(7,941)	19,223
Beginning balance (in shares) at Jun. 30, 2020		52,991
Beginning balance at Jun. 30, 2020	1,030,603	$ 5,299	811,321	204,726	(8,037)	17,294
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	24,557			22,628		1,929
Stock based compensation expense (in shares)		39
Stock based compensation expense	1,948		1,948
Common stock transactions related to stock based employee benefit plans (in shares)		18
Common stock transactions related to stock based employee benefit plans	96	$ 6	(6)		96
Common stock issued for stock options (in shares)		1
Common stock issued for stock options	16		16
Issuance of common stock, pursuant to acquisition (in shares)		2,120
Issuance of common stock, pursuant to acquisition	41,121	$ 212	40,909
Increase (decrease) in shares during period (in shares)		2,178
Increase (decrease) in stockholders' equity during period	67,738	$ 218	42,867	22,628	96	1,929
Ending balance (in shares) at Sep. 30, 2020		55,169
Ending balance at Sep. 30, 2020	$ 1,098,341	$ 5,517	$ 854,188	$ 227,354	$ (7,941)	$ 19,223