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)
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	36,549			51,313		(14,764)
Stock based compensation expense (in shares)		32
Stock based compensation expense	5,602		5,602
Common stock transactions related to stock based employee benefit plans (in shares)		(2)
Common stock transactions related to stock based employee benefit plans	(497)	$ 20	(22)		(495)
Common stock issued for stock options (in shares)		322
Common stock issued for stock options	1,513	$ 14	1,499
Increase (decrease) in shares during period (in shares)		352
Increase (decrease) in stockholders' equity during period	43,167	$ 34	7,079	51,198	(495)	(14,649)
Ending balance (in shares) at Sep. 30, 2018		47,270
Ending balance at Sep. 30, 2018	732,831	$ 4,727	668,711	81,112	(2,854)	(18,865)
Beginning balance (in shares) at Jun. 30, 2018		47,164
Beginning balance at Jun. 30, 2018	716,163	$ 4,716	665,885	64,790	(2,884)	(16,344)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	13,801			16,322		(2,521)
Stock based compensation expense (in shares)		32
Stock based compensation expense	1,980		1,980
Common stock transactions related to stock based employee benefit plans (in shares)		1
Common stock transactions related to stock based employee benefit plans	24	$ 3	(9)		30
Common stock issued for stock options (in shares)		73
Common stock issued for stock options	863	$ 8	855
Increase (decrease) in shares during period (in shares)		106
Increase (decrease) in stockholders' equity during period	16,668	$ 11	2,826	16,322	30	(2,521)
Ending balance (in shares) at Sep. 30, 2018		47,270
Ending balance at Sep. 30, 2018	732,831	$ 4,727	668,711	81,112	(2,854)	(18,865)
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