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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation (shares)	60,000
Balance, shares at Sep. 30, 2017	21,029,000
Balance at Dec. 31, 2016	$ 210,214	$ 210	$ 153,353	$ 53,640	$ 3,011
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of stock under stock-based compensation plan	968	0	968
Stock-based compensation	490		490
Dividends declared	(3,777)			(3,777)
Other comprehensive income (loss), net of tax	4,886				4,886
Net income	18,977			18,977
Balance at Sep. 30, 2017	231,758	210	154,811	68,840	7,897
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)	191,000
Balance, shares at Sep. 30, 2018	21,226,512
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	1,558	2	1,556
Stock-based compensation	1,083		1,083
Dividends declared	(4,439)			(4,439)
Other comprehensive income (loss), net of tax	(7,825)				(7,825)
Net income	29,339			29,339
Balance at Sep. 30, 2018	257,905	$ 212	$ 157,721	$ 103,584	$ (3,612)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustment - net unrealized gains on equity securities | Accounting Standards Update 2016-01 [Member]	$ 6,500