CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common stock amount	Additional paid-in capital	Retained earnings	Accumulated other comprehensive income (loss)	Treasury stock
Balance at beginning of period at Dec. 31, 2017	$ 354,641	$ 32,857	$ 186,702	$ 144,977	$ (4,554)	$ (5,341)
Comprehensive income:
Net income	7,812			7,812
Total other comprehensive income (loss)	(7,629)				(7,629)
Cash dividends declared	(2,884)			(2,884)
Net issuance to employee stock plans, including related tax effects	15		(112)			127
Modified retrospective basis adoption of Revenue Recognition Accounting Codification Standard 606				(184)
Modified retrospective basis adoption of Revenue Recognition Accounting Codification Standard 606 | Accounting Standards Update 2014-09	(184)
Reclassification of the income tax effects of the Tax Cuts and Jobs Act from accumulated other comprehensive income for adoption of ASU 2018-02				980	(980)
Recognition of stock based compensation	379		379
Balance at end of period at Mar. 31, 2018	352,150	32,857	186,969	150,701	(13,163)	(5,214)
Balance at beginning of period at Dec. 31, 2017	354,641	32,857	186,702	144,977	(4,554)	(5,341)
Comprehensive income:
Net income	16,347
Total other comprehensive income (loss)	(9,813)
Total comprehensive income	6,534
Balance at end of period at Jun. 30, 2018	355,963	32,857	187,198	156,139	(15,347)	(4,884)
Balance at beginning of period at Mar. 31, 2018	352,150	32,857	186,969	150,701	(13,163)	(5,214)
Comprehensive income:
Net income	8,535			8,535
Total other comprehensive income (loss)	(2,184)				(2,184)
Total comprehensive income	6,351
Cash dividends declared	(3,097)			(3,097)
Treasury stock purchased (9,294 shares)	(278)					(278)
Net issuance to employee stock plans, including related tax effects	589		(19)			608
Recognition of stock based compensation	248		248
Balance at end of period at Jun. 30, 2018	355,963	32,857	187,198	156,139	(15,347)	(4,884)
Balance at beginning of period at Dec. 31, 2018	370,579	32,857	187,653	166,526	(11,802)	(4,655)
Comprehensive income:
Net income	7,281			7,281
Total other comprehensive income (loss)	6,174				6,174
Cash dividends declared	(3,105)			(3,105)
Net issuance to employee stock plans, including related tax effects	(169)		(173)			4
Recognition of stock based compensation	263		263
Balance at end of period at Mar. 31, 2019	381,023	32,857	187,743	170,702	(5,628)	(4,651)
Balance at beginning of period at Dec. 31, 2018	370,579	32,857	187,653	166,526	(11,802)	(4,655)
Comprehensive income:
Net income	13,398
Total other comprehensive income (loss)	12,679
Total comprehensive income	26,077
Balance at end of period at Jun. 30, 2019	390,562	32,857	188,144	173,400	877	(4,716)
Balance at beginning of period at Mar. 31, 2019	381,023	32,857	187,743	170,702	(5,628)	(4,651)
Comprehensive income:
Net income	6,117			6,117
Total other comprehensive income (loss)	6,505				6,505
Total comprehensive income	12,622
Cash dividends declared	(3,419)			(3,419)
Treasury stock purchased (9,294 shares)	(210)					(210)
Net issuance to employee stock plans, including related tax effects	249		104			145
Recognition of stock based compensation	297		297
Balance at end of period at Jun. 30, 2019	$ 390,562	$ 32,857	$ 188,144	$ 173,400	$ 877	$ (4,716)