CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Thousands	Total	Common Stock	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning Balance (in shares) at Dec. 31, 2016		15,476,379
Beginning Balance at Dec. 31, 2016	$ 391,547	$ 156,041	$ 249,415	$ (13,909)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	31,649		31,649
Other comprehensive income, net of tax	1,398			1,398
Stock-based compensation expense	1,135	$ 1,135
Exercise of stock options and issuance of vested share awards, net of repurchase for tax withholdings and tax benefit (shares)		39,198
Exercise of stock options and issuance of vested share awards, net of repurchase for tax withholdings	(615)	$ (615)
Cash dividends declared ($0.46 and $0.55 per share	(10,748)		(10,748)
Ending Balance (in shares) at Sep. 30, 2017		15,515,577
Ending Balance at Sep. 30, 2017	414,366	$ 156,561	270,316	(12,511)
Beginning Balance (in shares) at Dec. 31, 2017		15,524,704
Beginning Balance at Dec. 31, 2017	403,413	$ 156,904	266,723	(20,214)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	39,094		39,094
Other comprehensive income, net of tax	(12,823)			(12,823)
Stock-based compensation expense	1,405	$ 1,405
Exercise of stock options and issuance of vested share awards, net of repurchase for tax withholdings and tax benefit (shares)		59,822
Exercise of stock options and issuance of vested share awards, net of repurchase for tax withholdings	(290)	$ (290)
Cash dividends declared ($0.46 and $0.55 per share	(13,274)		(13,274)
Ending Balance (in shares) at Sep. 30, 2018		15,584,526.000
Ending Balance at Sep. 30, 2018	$ 417,525	$ 158,019	292,741	(33,235)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative-effect adjustment (Note 2)			$ 198	$ (198)