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 Income (Loss) [Member]
Beginning Balance at Dec. 31, 2013	$ 416,814	$ 198	$ (22,370)	$ 192,537	$ 250,744	$ (4,295)
Beginning Balance, Shares at Dec. 31, 2013		19,855,430
Treasury Stock Beginning 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
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		20,155,936
Treasury Stock Ending Balance, Shares at Dec. 31, 2014			(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
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	20,388,396	20,388,396
Treasury Stock Ending Balance, Shares at Dec. 31, 2015	(1,258,250)		(1,258,250)
Comprehensive income:
Net income	$ 77,865				77,865
Other comprehensive income:
Change in unrealized gains loss, net of tax	(3,079)					(3,079)
Comprehensive income	74,786
Common stock issued upon exercise of options	837	$ 1		836
Common stock issued upon exercise of options, Shares		68,879
Restricted common stock issued	603			603
Restricted common stock issued, Shares		31,110
Share-based compensation	1,268			1,268
Tax benefit from share-based payments	995			995
Dividends to shareholders	(76,320)				(76,320)
Ending Balance at Dec. 31, 2016	$ 456,150	$ 204	$ (22,370)	$ 208,390	$ 270,418	$ (492)
Ending Balance, Shares at Dec. 31, 2016	20,488,385	20,488,385
Treasury Stock Ending Balance, Shares at Dec. 31, 2016	(1,258,250)		(1,258,250)