Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Common Stock [Member] Class A Common Stock [Member]	Common Stock [Member] Class B Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]
Balance at Dec. 31, 2014	$ 92,586	$ 0	$ 1	$ 0	$ 80,074	$ 7,297	$ 5,214
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issuance, net of transaction costs	90	0	0	0	90	0	0
Stock-based compensation	65	0	0	0	65	0	0
Other comprehensive income (loss), net of tax	(1,563)	0	0	0	0	0	(1,563)
Net income	22,273	0	0	0	0	22,273	0
Balance at Dec. 31, 2015	113,451	0	1	0	80,229	29,570	3,651
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of capital structure	0	160	(1)	0	(159)	0	0
Common stock issuance, net of transaction costs	72,841	50	0	0	72,791	0	0
Stock-based compensation	492	0	0	0	492	0	0
Dividends declared and paid	(2,097)	0	0	0	0	(2,097)	0
Other comprehensive income (loss), net of tax	(640)	0	0	0	0	0	(640)
Net income	26,167	0	0	0	0	26,167	0
Balance at Dec. 31, 2016	210,214	210	0	0	153,353	53,640	3,011
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under stock-based compensation plan	1,077				1,077
Stock-based compensation	652				652
Dividends declared and paid	(5,039)					(5,039)
Other comprehensive income (loss), net of tax	6,384						6,384
Net income	24,901					24,901
Balance at Dec. 31, 2017	$ 238,189	$ 210	$ 0	$ 0	$ 155,082	$ 73,502	$ 9,395