Quarterly Results (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 18,259	$ 18,194	$ 18,145	$ 17,750	$ 17,593	$ 16,801	$ 16,461	$ 16,226	$ 72,348	$ 67,081	$ 63,950
Net income before gain and excess insurance recovery proceeds	4,031	3,960	4,033	4,366					16,390
Gain on Arlington transaction				27,196					27,196
Hurricane insurance recovery proceeds in excess of damaged property write-downs	2,033								2,033
Net income	$ 6,064	$ 3,960	$ 4,033	$ 31,562	$ 4,446	$ 3,818	$ 4,523	$ 4,428	$ 45,619	$ 17,215	$ 23,691
Total basic earnings per share	$ 0.44	$ 0.29	$ 0.30	$ 2.32	$ 0.33	$ 0.28	$ 0.34	$ 0.33	$ 3.35	$ 1.28	$ 1.78
Total diluted earnings per share	$ 0.44	$ 0.29	$ 0.30	$ 2.32	$ 0.33	$ 0.28	$ 0.34	$ 0.33	$ 3.35	$ 1.28	$ 1.78