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, 2014	$ 361,296	$ 96,121	$ 263,345	$ 3,830	$ (2,000)
Balance (in shares) at Dec. 31, 2014		24,698,431
Net income	46,367		46,367
Other comprehensive income (loss), net of tax	(1,688)			(1,688)
Comprehensive income (loss)	44,679
Cash dividends declared	(15,710)		(15,710)
Treasury shares purchased under deferred directors' plan	0	$ 455			(455)
Treasury shares purchased under deferred directors' plan (in shares)		(16,356)
Stock activity under equity incentive plans	12	$ 12
Stock activity under equity incentive plans (in shares)		136,991
Stock based compensation expense	2,535	$ 2,535
Balance at Dec. 31, 2015	392,812	$ 99,123	294,002	2,142	(2,455)
Balance (in shares) at Dec. 31, 2015		24,819,066
Net income	52,084		52,084
Other comprehensive income (loss), net of tax	(4,529)			(4,529)
Comprehensive income (loss)	47,555
Cash dividends declared	(18,213)		(18,213)
Treasury shares purchased under deferred directors' plan	0	$ 458			(458)
Treasury shares purchased under deferred directors' plan (in shares)		(14,811)
Stock activity under equity incentive plans	614	$ 614
Stock activity under equity incentive plans (in shares)		133,646
Stock based compensation expense	4,210	$ 4,210
Fractional shares retired due to 3-for-2 stock split (in shares)		(36)
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	57,330		57,330
Other comprehensive income (loss), net of tax	1,717			1,717
Comprehensive income (loss)	59,047
Cash dividends declared	(21,409)		(21,409)
Treasury shares purchased under deferred directors' plan	0	$ 495			(495)
Treasury shares purchased under deferred directors' plan (in shares)		(10,748)
Stock activity under equity incentive plans	(1,736)	$ (1,736)
Stock activity under equity incentive plans (in shares)		98,816
Stock based compensation expense	5,698	$ 5,698
Balance at Dec. 31, 2017	$ 468,578	$ 108,862	$ 363,794	$ (670)	$ (3,408)
Balance (in shares) at Dec. 31, 2017		25,025,933