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	$ 689,663	$ 4,693	$ 661,632	$ 29,913	$ (2,359)	$ (4,216)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	10,018			18,027		(8,009)
Stock based compensation expense	1,455		1,455
Common stock issued for stock based employee benefit plans (in shares)		3
Common stock issued for stock based employee benefit plans	78		(2)		80
Common stock issued for stock options (in shares)		62
Common stock issued for stock options	647	$ 5	642
Increase (decrease) in shares during the period (in shares)		65
Period increase (decrease) in stockholders' equity	12,198	$ 5	2,095	17,912	80	(7,894)
Ending balance (in shares) at Mar. 31, 2018		46,983
Ending balance at Mar. 31, 2018	701,861	$ 4,698	663,727	47,825	(2,279)	(12,110)
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	31,548			22,705		8,843
Stock based compensation expense	2,129		2,129
Common stock issued for stock based employee benefit plans (in shares)		49
Common stock issued for stock based employee benefit plans	(1,584)	$ 5	(14)		(1,575)
Common stock issued for stock options (in shares)		4
Common stock issued for stock options	64	$ 0	64
Increase (decrease) in shares during the period (in shares)		53
Period increase (decrease) in stockholders' equity	32,157	$ 5	2,179	22,705	(1,575)	8,843
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)