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, 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
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	80,411		80,411
Other comprehensive income (loss), net of tax	(5,280)			(5,280)
Comprehensive income (loss)	75,131
Cash dividends declared	(25,291)		(25,291)
Treasury shares purchased under deferred directors' plan	0	$ 463			(463)
Treasury shares purchased under deferred directors' plan (in shares)		(9,625)
Treasury shares sold and distributed under deferred directors' plan	0	$ (115)			115
Treasury shares sold and distributed under deferred directors' plan (in shares)		5,636
Stock activity under equity incentive plans	(2,435)	$ (2,435)
Stock activity under equity incentive plans (in shares)		106,829
Stock based compensation expense	5,608	$ 5,608
Balance at Dec. 31, 2018	$ 521,615	$ 112,383	$ 419,179	$ (6,191)	$ (3,756)
Balance (in shares) at Dec. 31, 2018		25,128,773