Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]
Balance, shares at Dec. 31, 2016	20,969,000
Balance, shares at Jun. 30, 2017	20,969,000
Balance at Dec. 31, 2016	$ 210,214	$ 210	$ 153,353	$ 53,640	$ 3,011
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	324		324
Dividends declared	(2,516)			(2,516)
Other comprehensive income (loss), net of tax	3,247				3,247
Net income	14,776			14,776
Balance at Jun. 30, 2017	226,045	210	153,677	65,900	6,258
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustment - net unrealized gains on equity securities | Accounting Standards Update 2016-01 [Member]	$ 0			6,490	(6,490)
Balance, shares at Dec. 31, 2017	21,036,087
Balance, shares (Accounting Standards Update 2016-01 [Member]) at Dec. 31, 2017	21,036,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation (shares)	133,000
Balance, shares at Jun. 30, 2018	21,169,384
Balance at Dec. 31, 2017	$ 238,189	210	155,082	73,502	9,395
Balance (Accounting Standards Update 2016-01 [Member]) at Dec. 31, 2017	238,189	210	155,082	79,992	2,905
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of tax effects resulting from the TCJA	0			(1,308)	1,308
Issuance of stock under stock-based compensation plan	624	1	623
Stock-based compensation	559		559
Dividends declared	(2,954)			(2,954)
Other comprehensive income (loss), net of tax	(5,872)				(5,872)
Net income	17,399			17,399
Balance at Jun. 30, 2018	247,945	$ 211	$ 156,264	$ 93,129	$ (1,659)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustment - net unrealized gains on equity securities | Accounting Standards Update 2016-01 [Member]	$ 6,500