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, 2016	$ 456,150	$ 204	$ 208,390	$ (22,370)	$ 270,418	$ (492)
Beginning Balance, Shares at Dec. 31, 2016		20,488,385
Treasury Stock Beginning Balance, Shares at Dec. 31, 2016				(1,258,250)
Comprehensive income:
Net income	46,231				46,231
Other comprehensive income:
Change in unrealized gains (losses), net of tax	4,104					4,104
Comprehensive income	50,335
Common stock issued	396		396
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
Treasury Stock Ending Balance, Shares at Dec. 31, 2017				(1,258,250)
Impact of adoption of ASU | ASU 2016-01 [Member]					615	(615)
Impact of adoption of ASU | ASU 2018-02 [Member]					(414)	414
Comprehensive income:
Net income	71,632				71,632
Other comprehensive income:
Change in unrealized gains (losses), net of tax	(4,243)					(4,243)
Comprehensive income	67,389
Common stock issued upon exercise of options	67		67
Common stock issued upon exercise of options, Shares		15,000
Common stock issued	196	$ 1	195
Common stock issued, Shares		9,065
Share-based compensation	1,088		1,088
Dividends to shareholders	(84,401)				(84,401)
Ending Balance at Dec. 31, 2018	$ 409,762	$ 205	211,431	$ (22,370)	221,328	(832)
Ending Balance, Shares at Dec. 31, 2018	19,269,980	20,528,230
Treasury Stock Ending Balance, Shares at Dec. 31, 2018	(1,258,250)			(1,258,250)
Impact of adoption of ASU | ASU 2016-02 [Member]	$ (1)				(1)
Comprehensive income:
Net income	92,690				92,690
Other comprehensive income:
Change in unrealized gains (losses), net of tax	13,043					13,043
Comprehensive income	105,733
Common stock issued upon exercise of options	20		20
Common stock issued upon exercise of options, Shares		5,000
Common stock issued	560		560
Common stock issued, Shares		27,603
Share-based compensation	993		993
Dividends to shareholders	(86,852)				(86,852)
Ending Balance at Dec. 31, 2019	$ 430,215	$ 205	$ 213,004	$ (22,370)	$ 227,165	$ 12,211
Ending Balance, Shares at Dec. 31, 2019	19,302,583	20,560,833
Treasury Stock Ending Balance, Shares at Dec. 31, 2019	(1,258,250)			(1,258,250)