Consolidated Statements of Changes in Stockholders' Equity Statement - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income	Noncontrolling Interest
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principle, net of tax effect	$ 0			$ 2,203	$ (2,203)
Beginning balance at Dec. 31, 2017	463,209	$ 21	$ 84,866	376,119	2,203	$ 0
Beginning balance (in shares) at Dec. 31, 2017		20,520,856
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income before noncontrolling interest	1,285			1,285
Share-based compensation	457		487	(30)
Issuance of restricted and deferred stock units	(2,020)		(2,020)
Issuance of restricted and deferred stock units (in shares)		51,442
Employee stock purchase plan	653		653
Employee stock purchase plan (in shares)		12,838
Dividends paid	(5,754)			(5,754)
Ending balance at Mar. 31, 2018	457,830	$ 21	83,986	373,823	0	0
Ending balance (in shares) at Mar. 31, 2018		20,585,136
Beginning balance at Dec. 31, 2017	463,209	$ 21	84,866	376,119	2,203	0
Beginning balance (in shares) at Dec. 31, 2017		20,520,856
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income before noncontrolling interest	29,944
Ending balance at Sep. 30, 2018	474,957	$ 21	84,045	390,891	0	0
Ending balance (in shares) at Sep. 30, 2018		20,631,171
Beginning balance at Mar. 31, 2018	457,830	$ 21	83,986	373,823	0	0
Beginning balance (in shares) at Mar. 31, 2018		20,585,136
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income before noncontrolling interest	12,871			12,871
Share-based compensation	362		392	(30)
Issuance of restricted and deferred stock units	(3)		(3)
Issuance of restricted and deferred stock units (in shares)		9,350
Dividends paid	(5,766)			(5,766)
Ending balance at Jun. 30, 2018	465,294	$ 21	84,375	380,898	0	0
Ending balance (in shares) at Jun. 30, 2018		20,594,486
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income before noncontrolling interest	15,788			15,788
Share-based compensation	486		504	(18)
Issuance of restricted and deferred stock units	(1,552)		(1,552)
Issuance of restricted and deferred stock units (in shares)		23,616
Employee stock purchase plan	718		718
Employee stock purchase plan (in shares)		13,069
Dividends paid	(5,777)			(5,777)
Ending balance at Sep. 30, 2018	474,957	$ 21	84,045	390,891	0	0
Ending balance (in shares) at Sep. 30, 2018		20,631,171
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principle, net of tax effect	97			97
Beginning balance at Dec. 31, 2018	$ 889,312	$ 28	495,366	393,918	0	0
Beginning balance (in shares) at Dec. 31, 2018	28,404,316	28,404,316
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income before noncontrolling interest	$ 5,873			5,873
Share-based compensation	870		886	(16)
Issuance of restricted and deferred stock units	(132)		(132)
Issuance of restricted and deferred stock units (in shares)		14,312
Employee stock purchase plan	811		811
Employee stock purchase plan (in shares)		15,932
Common stock issuance cost	(10)		(10)
Dividends paid	(8,528)			(8,528)
Ending balance at Mar. 31, 2019	888,293	$ 28	496,921	391,344	0	0
Ending balance (in shares) at Mar. 31, 2019		28,434,560
Beginning balance at Dec. 31, 2018	$ 889,312	$ 28	495,366	393,918	0	0
Beginning balance (in shares) at Dec. 31, 2018	28,404,316	28,404,316
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income before noncontrolling interest	$ 29,113
Ending balance at Sep. 30, 2019	$ 896,664	$ 28	499,377	397,259	0	0
Ending balance (in shares) at Sep. 30, 2019	28,456,490	28,456,490
Beginning balance at Mar. 31, 2019	$ 888,293	$ 28	496,921	391,344	0	0
Beginning balance (in shares) at Mar. 31, 2019		28,434,560
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income before noncontrolling interest	13,762			13,538		224
Distribution to noncontrolling interest	(224)					(224)
Share-based compensation	702		718	(16)
Issuance of restricted and deferred stock units	(6)		(6)
Issuance of restricted and deferred stock units (in shares)		7,579
Dividends paid	(8,532)			(8,532)
Ending balance at Jun. 30, 2019	893,995	$ 28	497,633	396,334	0	0
Ending balance (in shares) at Jun. 30, 2019		28,442,139
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income before noncontrolling interest	9,478			9,478
Share-based compensation	936		952	(16)
Issuance of restricted and deferred stock units	0
Employee stock purchase plan	792		792
Employee stock purchase plan (in shares)		14,351
Dividends paid	(8,537)			(8,537)
Ending balance at Sep. 30, 2019	$ 896,664	$ 28	$ 499,377	$ 397,259	$ 0	$ 0
Ending balance (in shares) at Sep. 30, 2019	28,456,490	28,456,490