Quarterly Financial Data - Summary of Quarterly Results of Operations (Detail) - USD ($)		3 Months Ended								12 Months Ended
	Aug. 03, 2015	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues from rental properties	$ 15,000,000	$ 29,452,000	$ 29,570,000	$ 29,022,000	$ 28,284,000	$ 28,158,000	$ 24,913,000	$ 24,364,000	$ 23,897,000	$ 116,328,000	$ 101,332,000	$ 96,711,000
Earnings from continuing operations		13,408,000	11,017,000	13,823,000	10,162,000	13,321,000	9,460,000	14,551,000	7,716,000	48,410,000	45,048,000	39,825,000
Net earnings		$ 13,190,000	$ 10,944,000	$ 13,540,000	$ 10,032,000	$ 13,036,000	$ 9,340,000	$ 15,106,000	$ 9,704,000	$ 47,706,000	$ 47,186,000	$ 38,411,000
Diluted earnings per common share:
Earnings from continuing operations		$ 0.33	$ 0.27	$ 0.34	$ 0.25	$ 0.33	$ 0.24	$ 0.41	$ 0.22	$ 1.19	$ 1.20	$ 1.16
Net earnings		$ 0.32	$ 0.27	$ 0.33	$ 0.25	$ 0.33	$ 0.24	$ 0.43	$ 0.28	$ 1.17	$ 1.26	$ 1.12