CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Treasury Stock [Member]
Balance at Dec. 31, 2016	$ 426,978	$ 104,405	$ 327,873	$ (2,387)	$ (2,913)
Balance (in shares) at Dec. 31, 2016		24,937,865
Net income	29,878		29,878
Other comprehensive income (loss), net of tax	2,756			2,756
Cash dividends declared	(10,324)		(10,324)
Treasury shares purchased under deferred directors' plan	0	$ 256			(256)
Treasury shares purchased under deferred directors' plan (in shares)		(5,638)
Stock activity under equity compensation plans	(1,736)	$ (1,736)
Stock activity under equity compensation plans (in shares)		89,532
Stock based compensation expense	2,819	$ 2,819
Balance at Jun. 30, 2017	450,371	$ 105,744	347,427	369	(3,169)
Balance (in shares) at Jun. 30, 2017		25,021,759
Balance at Dec. 31, 2017	468,578	$ 108,862	363,794	(670)	(3,408)
Balance (in shares) at Dec. 31, 2017		25,025,933
Adoption of ASU | Accounting Standards Update 2018-02 [Member]	0		173	(173)
Adoption of ASU | Accounting Standards Update 2014-09 [Member]	24		24
Adoption of ASU | Accounting Standards Update 2016-01 [Member]	0		68	(68)
Net income	38,478		38,478
Other comprehensive income (loss), net of tax	(8,799)			(8,799)
Cash dividends declared	(12,133)		(12,133)
Treasury shares purchased under deferred directors' plan	0	$ 229			(229)
Treasury shares purchased under deferred directors' plan (in shares)		(4,711)
Treasury stock sold and distributed under deferred directors' plan	0	$ (115)			115
Treasury stock sold and distributed under deferred directors' plan (in shares)		5,636
Stock activity under equity compensation plans	(2,435)	$ (2,435)
Stock activity under equity compensation plans (in shares)		99,679
Stock based compensation expense	2,682	$ 2,682
Balance at Jun. 30, 2018	$ 486,395	$ 109,223	$ 390,404	$ (9,710)	$ (3,522)
Balance (in shares) at Jun. 30, 2018		25,126,537