Investments in Mortgage Revenue Bonds - Schedule of MRBs Acquisitions (Details)		12 Months Ended
		Dec. 31, 2019 USD ($) Unit	Dec. 31, 2018 USD ($) Unit
Schedule Of Available For Sale Securities [Line Items]
Units | Unit		3,174
Principal Outstanding at Date of Acquisition | $		$ 19,250,000	$ 41,708,000
Gateway Village [Member] | Durham NC [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Acquired		February
Units | Unit		64
Maturity Date		Apr. 01, 2032
Base Interest Rate		6.10%
Principal Outstanding at Date of Acquisition | $		$ 2,600,000
Lynnhaven Apartments [Member] | Durham NC [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Acquired		February
Units | Unit		75
Maturity Date		Apr. 01, 2032
Base Interest Rate		6.10%
Principal Outstanding at Date of Acquisition | $		$ 3,450,000
Montevista [Member] | Series A [Member] | San Pablo, CA [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Acquired		June
Units | Unit		82
Maturity Date		Jul. 01, 2036
Base Interest Rate		5.75%
Principal Outstanding at Date of Acquisition | $		$ 6,720,000
Montevista [Member] | Series B [Member] | San Pablo, CA [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Acquired		June
Units | Unit		82
Maturity Date		Jul. 01, 2021
Base Interest Rate		5.75%
Principal Outstanding at Date of Acquisition | $		$ 6,480,000
Esperanza at Palo Alto [Member] | San Antonio, TX [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Acquired	[1]		May
Units | Unit	[1]		322
Maturity Date	[1]		Jul. 01, 2058
Base Interest Rate	[1]		5.80%
Principal Outstanding at Date of Acquisition | $	[1]		$ 19,540,000
Solano Vista [Member] | Series A [Member] | Vallejo, CA [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Acquired			December
Units | Unit			96
Maturity Date			Jan. 01, 2036
Base Interest Rate			5.85%
Principal Outstanding at Date of Acquisition | $			$ 2,665,000
Solano Vista [Member] | Series B [Member] | Vallejo, CA [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Acquired			December
Units | Unit			96
Maturity Date			Jan. 01, 2021
Base Interest Rate			5.85%
Principal Outstanding at Date of Acquisition | $			$ 3,103,000
Village at Avalon [Member] | Albuquerque, NM [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Acquired	[1]		December
Units | Unit	[1]		240
Maturity Date	[1]		Jan. 01, 2059
Base Interest Rate	[1]		5.80%
Principal Outstanding at Date of Acquisition | $	[1]		$ 16,400,000

[1] Previously reported bond purchase commitment that converted to an MRB.