Discontinued Operations and Assets Held For Sale - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Property Portfolio	Dec. 31, 2016 USD ($) Portfolio	Dec. 31, 2015 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold previously held for sale | Portfolio		2
(Loss) gains from dispositions of real estate		$ (178,000)	$ 339,000
Gain from sale of properties	$ 1,025,000	$ 2,373,000
Number of whole properties sold not meeting criteria of discontinued operations recognized gain included in continuing operations | Portfolio	12	12
Number of partial properties sold not meeting criteria of discontinued operations recognized gain included in continuing operations | Portfolio	1
Gain on property condemnations	$ 16,000	$ 177,000
Number of real estate properties held and used | Property	2
Properties Previously Classified Held for Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
(Loss) gains from dispositions of real estate		(205,000)
Ramoco Affiliates [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Deferred gain on sale of property		$ 3,868,000