Real Estate - Schedule of Property Dispositions (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on disposition of properties, net of tax	$ 2,014
Disposal Group, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale Price	10,500	$ 66,500
Gain on disposition of properties, net of tax	$ 2,014	$ 5,140
Fund III | Disposal Group, Not Discontinued Operations | A3104 M St. Washington D C
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold	Jan. 24, 2019
Sale Price	$ 10,500
Gain on disposition of properties, net of tax	$ 2,014
Fund II | Disposal Group, Not Discontinued Operations | Sherman Avenue
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold		Apr. 17, 2018
Sale Price		$ 26,000
Gain on disposition of properties, net of tax		$ 33
Fund IV | Disposal Group, Not Discontinued Operations | Lake Montclair
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold		Aug. 27, 2018
Sale Price		$ 22,450
Gain on disposition of properties, net of tax		$ 2,923
Fund IV | Disposal Group, Not Discontinued Operations | 1861 Union Street
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold		Aug. 29, 2018
Sale Price		$ 6,000
Gain on disposition of properties, net of tax		$ 2,184
Fund IV | Disposal Group, Not Discontinued Operations | Bowery4 Residential Condos New York N Y | Scenario One
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold		Nov. 30, 2018
Fund IV | Disposal Group, Not Discontinued Operations | Bowery4 Residential Condos New York N Y | Scenario Two
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold		Dec. 10, 2018
Fund IV | Disposal Group, Not Discontinued Operations | Bowery4 Residential Condos New York N Y | Scenario Three
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold		Dec. 17, 2018
Fund IV | Disposal Group, Not Discontinued Operations | Bowery4 Residential Condos New York N Y | Scenario Four
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold		Dec. 21, 2018
Sale Price		$ 12,050