Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	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, 2012	$ 381,222	$ 195	$ (22,370)	$ 187,401	$ 213,017	$ 2,979
Beginning Balance, Shares at Dec. 31, 2012		19,513,476
Treasury Stock Beginning Balance, Shares at Dec. 31, 2012			(1,258,250)
Comprehensive income:
Net income	43,637				43,637
Other comprehensive income:
Change in unrealized gains loss, 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 from share-based payments	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		19,855,430
Treasury Stock Ending Balance, Shares at Dec. 31, 2013			(1,258,250)
Comprehensive income:
Net income	53,666				53,666
Other comprehensive income:
Change in unrealized gains loss, 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 from share-based payments	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
Treasury Stock Ending Balance, Shares at Dec. 31, 2014	(1,258,250)		(1,258,250)
Comprehensive income:
Net income	$ 70,462				70,462
Other comprehensive income:
Change in unrealized gains loss, net of tax	(223)					(223)
Comprehensive income	70,239
Common stock issued upon exercise of options	$ 1,844	$ 2		1,842
Common stock issued upon exercise of options, Shares	193,204	193,204
Restricted common stock issued	$ 502			502
Restricted common stock issued, Shares		39,256
Share-based compensation	1,003			1,003
Tax benefit from share-based payments	2,203			2,203
Dividends to shareholders	(68,778)				(68,778)
Ending Balance at Dec. 31, 2015	$ 453,981	$ 203	$ (22,370)	$ 204,688	$ 268,873	$ 2,587
Ending Balance, Shares at Dec. 31, 2015	19,130,146	20,388,396
Treasury Stock Ending Balance, Shares at Dec. 31, 2015	(1,258,250)		(1,258,250)