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), Net
Beginning balance (in shares) at Dec. 31, 2017		46,918
Beginning balance at Dec. 31, 2017	$ 689,664	$ 4,693	$ 661,632	$ 29,914	$ (2,359)	$ (4,216)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	22,748			34,991		(12,243)
Stock based compensation expense	3,622		3,622
Common stock issued for stock based employee benefit plans (in shares)		(3)
Common stock issued for stock based employee benefit plans	(521)	$ 17	(13)		(525)
Common stock issued for stock options (in shares)		249
Common stock issued for stock options	650	$ 6	644
Increase (decrease) in shares during period (in shares)		246
Increase (decrease) in stockholders' equity during period	26,499	$ 23	4,253	34,876	(525)	(12,128)
Ending balance (in shares) at Jun. 30, 2018		47,164
Ending balance at Jun. 30, 2018	716,163	$ 4,716	665,885	64,790	(2,884)	(16,344)
Beginning balance (in shares) at Mar. 31, 2018		46,983
Beginning balance at Mar. 31, 2018	701,862	$ 4,698	663,727	47,826	(2,279)	(12,110)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	12,730			16,964		(4,234)
Stock based compensation expense	2,167		2,167
Common stock issued for stock based employee benefit plans (in shares)		(6)
Common stock issued for stock based employee benefit plans	(599)	$ 17	(11)		(605)
Common stock issued for stock options (in shares)		187
Common stock issued for stock options	3	$ 1	2
Increase (decrease) in shares during period (in shares)		181
Increase (decrease) in stockholders' equity during period	14,301	$ 18	2,158	16,964	(605)	(4,234)
Ending balance (in shares) at Jun. 30, 2018		47,164
Ending balance at Jun. 30, 2018	716,163	$ 4,716	665,885	64,790	(2,884)	(16,344)
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]
Common stock issued for stock based employee benefit plans (in shares)		22
Ending balance (in shares) at Mar. 31, 2019		51,414
Ending balance at Mar. 31, 2019	896,424	$ 5,141	780,680	119,779	(4,959)	(4,217)
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 issued for stock based employee benefit plans (in shares)		71
Common stock issued for 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 issued for 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