Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Treasury Stock [Member]	Accumulated Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning Balance at Dec. 31, 2014	$ 446,968	$ 201	$ 199,138	$ (22,370)	$ 267,189	$ 2,810
Beginning Balance, Shares at Dec. 31, 2014		20,155,936
Treasury Stock Beginning 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	204,688	$ (22,370)	268,873	2,587
Ending Balance, Shares at Dec. 31, 2015		20,388,396
Treasury Stock Ending Balance, Shares at Dec. 31, 2015				(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	208,390	$ (22,370)	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)
Comprehensive income:
Net income	$ 46,231				46,231
Other comprehensive income:
Change in unrealized gains loss, net of tax	4,104					4,104
Comprehensive income	50,335
Restricted common stock issued	396		396
Restricted common stock issued, Shares		15,780
Share-based compensation	1,295		1,295
Dividends to shareholders	(82,753)				(82,753)
Ending Balance at Dec. 31, 2017	$ 425,423	$ 204	$ 210,081	$ (22,370)	$ 233,896	$ 3,612
Ending Balance, Shares at Dec. 31, 2017	20,504,165	20,504,165
Treasury Stock Ending Balance, Shares at Dec. 31, 2017	(1,258,250)			(1,258,250)