Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Employee Stock Ownership Plan	Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Loss	Treasury Stock	Common Stock Acquired by Deferred Compensation Plan	Deferred Compensation Plan Liability
Beginning Balance at Dec. 31, 2015	$ 238,446	$ (3,045)	$ 336	$ 269,757	$ 229,140	$ (6,241)	$ (251,501)	$ (314)	$ 314
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	7,866				7,866
Other comprehensive income, net of tax	443					443
Tax expense of stock plans	(260)			(260)
Stock awards	756			756
Treasury stock allocated to restricted stock plan				1,108	(114)		(994)
Issued 8,282,296 treasury shares to finance acquisition	165,901			36,940			128,961
Allocation of ESOP stock	301	142		159
Cash dividend	(5,481)				(5,481)
Exercise of stock options	1,286				(516)		1,802
Sale/Purchase of stock for the deferred compensation plan								6	(6)
Ending Balance at Jun. 30, 2016	409,258	(2,903)	336	308,460	230,895	(5,798)	(121,732)	(308)	308
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Effect of adopting Accounting Standards Update (ASU) No. 2016-09				(11,129)	11,129
Beginning Balance at Dec. 31, 2016	572,038	(2,761)	336	364,433	238,192	(5,614)	(22,548)	(313)	313
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	19,698
Other comprehensive income, net of tax	416					416
Stock awards	1,204			1,204
Treasury stock allocated to restricted stock plan				(1,544)	747		797
Allocation of ESOP stock	473	141		332
Cash dividend	(9,557)				(9,557)
Exercise of stock options	3,031				(1,739)		4,770
Sale/Purchase of stock for the deferred compensation plan								137	(137)
Ending Balance at Jun. 30, 2017	$ 587,303	$ (2,620)	$ 336	$ 353,296	$ 258,470	$ (5,198)	$ (16,981)	$ (176)	$ 176