Consolidated Statements of Changes in Shareholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Earnings [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning Balance at Dec. 31, 2011	$ 349,437	$ 194	$ (22,370)	$ 185,734	$ 183,664	$ 2,215
Beginning Balance, Shares at Dec. 31, 2011		19,408,512	(1,258,250)
Comprehensive income:
Net income	29,353				29,353
Other comprehensive income:
Change in unrealized gains, net of tax	764					764
Comprehensive income	30,117
Common stock issued upon exercise of options	781	1		780
Common stock issued upon exercise of options, Shares	86,800	86,800
Restricted common stock issued	240			240
Restricted common stock issued, Shares		18,164
Share-based compensation	226			226
Tax benefit of share-based compensation	421			421
Ending Balance at Dec. 31, 2012	381,222	195	(22,370)	187,401	213,017	2,979
Ending Balance, Shares at Dec. 31, 2012		19,513,476	(1,258,250)
Comprehensive income:
Net income	43,637				43,637
Other comprehensive income:
Change in unrealized gains, net of tax	(7,274)					(7,274)
Comprehensive income	36,363
Common stock issued upon exercise of options	2,344	3		2,341
Common stock issued upon exercise of options, Shares	256,300	256,300
Restricted common stock issued	557			557
Restricted common stock issued, Shares		85,654
Share-based compensation	214			214
Tax benefit of share-based compensation	2,024			2,024
Dividends to shareholders	(5,910)				(5,910)
Ending Balance at Dec. 31, 2013	416,814	198	(22,370)	192,537	250,744	(4,295)
Ending Balance, Shares at Dec. 31, 2013	18,597,180	19,855,430	(1,258,250)
Comprehensive income:
Net income	53,666				53,666
Other comprehensive income:
Change in unrealized gains, net of tax	7,105					7,105
Comprehensive income	60,771
Common stock issued upon exercise of options	2,674	3		2,671
Common stock issued upon exercise of options, Shares	294,165	294,165
Restricted common stock issued, Shares		6,341
Share-based compensation	1,089			1,089
Tax benefit of share-based compensation	2,841			2,841
Dividends to shareholders	(37,221)				(37,221)
Ending Balance at Dec. 31, 2014	$ 446,968	$ 201	$ (22,370)	$ 199,138	$ 267,189	$ 2,810
Ending Balance, Shares at Dec. 31, 2014	18,897,686	20,155,936	(1,258,250)