Condensed Consolidated Statement of Changes in Shareholders' Equity (Unaudited) - 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						$ (492)
Comprehensive income	$ 49,107
Common stock issued upon exercise of options, Shares	0
Ending Balance at Sep. 30, 2017						3,033
Beginning Balance at Jun. 30, 2017						2,416
Comprehensive income	$ 17,194
Ending Balance at Sep. 30, 2017						3,033
Beginning Balance at Dec. 31, 2017	$ 425,423	$ 204	$ 210,081	$ (22,370)	$ 233,896	3,612
Beginning Balance, Shares at Dec. 31, 2017	20,504,165	20,504,165
Treasury Stock Beginning Balance, Shares at Dec. 31, 2017	(1,258,250)			(1,258,250)
Comprehensive income	$ 45,013				52,826	(7,813)
Impact of adoption of ASU | ASU 2016-01 [Member]	614				614
Impact of adoption of ASU | ASU 2018-02 [Member]	(414)				(414)
Common stock issued upon exercise of options	$ 67		67
Common stock issued upon exercise of options, Shares	15,000	15,000
Common stock issued	$ 196	$ 1	195
Common stock issued, Shares		9,065
Share-based compensation	813		813
Dividends to shareholders	(12,716)				(12,716)
Ending Balance at Sep. 30, 2018	$ 458,996	$ 205	211,156	$ (22,370)	274,206	(4,201)
Ending Balance, Shares at Sep. 30, 2018	20,528,230	20,528,230
Treasury Stock Ending Balance, Shares at Sep. 30, 2018	(1,258,250)			(1,258,250)
Beginning Balance at Jun. 30, 2018						(2,083)
Comprehensive income	$ 17,583
Ending Balance at Sep. 30, 2018	$ 458,996	$ 205	$ 211,156	$ (22,370)	$ 274,206	$ (4,201)
Ending Balance, Shares at Sep. 30, 2018	20,528,230	20,528,230
Treasury Stock Ending Balance, Shares at Sep. 30, 2018	(1,258,250)			(1,258,250)