Quarterly Results (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 17,593	$ 16,801	$ 16,461	$ 16,226	$ 16,013	$ 15,686	$ 16,049	$ 16,202	$ 67,081	$ 63,950	$ 59,786
Net income before gains					4,352	3,639	3,262	3,696		14,949
Gain on property exchange							8,742			8,742
Net income	$ 4,446	$ 3,818	$ 4,523	$ 4,428	$ 4,352	$ 3,639	$ 12,004	$ 3,696	$ 17,215	$ 23,691	$ 51,551
Total basic earnings per share	$ 0.33	$ 0.28	$ 0.34	$ 0.33	$ 0.33	$ 0.27	$ 0.90	$ 0.28	$ 1.28	$ 1.78	$ 3.99
Total diluted earnings per share	$ 0.33	$ 0.28	$ 0.34	$ 0.33	$ 0.33	$ 0.27	$ 0.90	$ 0.28	$ 1.28	$ 1.78	$ 3.99