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, 2015						$ 2,587
Comprehensive income	$ 62,801
Ending Balance at Sep. 30, 2016						6,596
Beginning Balance at Jun. 30, 2016						8,309
Comprehensive income	16,183
Ending Balance at Sep. 30, 2016						6,596
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	20,488,385
Treasury Stock Beginning Balance, Shares at Dec. 31, 2016	(1,258,250)			(1,258,250)
Comprehensive income	$ 49,107				45,582	3,525
Restricted common stock issued	396		396
Restricted common stock issued, Shares		13,888
Share-based compensation	836		836
Dividends to shareholders	(11,544)				(11,544)
Ending Balance at Sep. 30, 2017	$ 494,945	$ 204	209,622	$ (22,370)	304,456	3,033
Ending Balance, Shares at Sep. 30, 2017	20,502,273	20,502,273
Treasury Stock Ending Balance, Shares at Sep. 30, 2017	(1,258,250)			(1,258,250)
Beginning Balance at Jun. 30, 2017						2,416
Comprehensive income	$ 17,194
Ending Balance at Sep. 30, 2017	$ 494,945	$ 204	$ 209,622	$ (22,370)	$ 304,456	$ 3,033
Ending Balance, Shares at Sep. 30, 2017	20,502,273	20,502,273
Treasury Stock Ending Balance, Shares at Sep. 30, 2017	(1,258,250)			(1,258,250)