Investments in Mortgage Revenue Bonds ("MRBs") - Schedule of MRB Acquisitions (Details)		6 Months Ended
		Jun. 30, 2018 USD ($) Unit	Jun. 30, 2017 USD ($) Unit
Schedule Of Available For Sale Securities [Line Items]
Principal Outstanding at Date of Acquisition		$ 19,540,000	$ 59,585,000
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
Avistar at Copperfield [Member] | Houston, Texas [Member] | Series A [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Acquired			February
Units | Unit			192
Maturity Date			May 01, 2054
Base Interest Rate			5.75%
Principal Outstanding at Date of Acquisition			$ 10,000,000
Avistar at Copperfield [Member] | Houston, Texas [Member] | Series B [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Acquired			February
Units | Unit			192
Maturity Date			Jun. 01, 2054
Base Interest Rate			12.00%
Principal Outstanding at Date of Acquisition			$ 4,000,000
Avistar at Wilcrest [Member] | Houston, Texas [Member] | Series A [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Acquired			February
Units | Unit			88
Maturity Date			May 01, 2054
Base Interest Rate			5.75%
Principal Outstanding at Date of Acquisition			$ 3,775,000
Avistar at Wilcrest [Member] | Houston, Texas [Member] | Series B [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Acquired			February
Units | Unit			88
Maturity Date			Jun. 01, 2054
Base Interest Rate			12.00%
Principal Outstanding at Date of Acquisition			$ 1,550,000
Avistar at Wood Hollow [Member] | Austin, Texas [Member] | Series A [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Acquired			February
Units | Unit			409
Maturity Date			May 01, 2054
Base Interest Rate			5.75%
Principal Outstanding at Date of Acquisition			$ 31,850,000
Avistar at Wood Hollow [Member] | Austin, Texas [Member] | Series B [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Acquired			February
Units | Unit			409
Maturity Date			Jun. 01, 2054
Base Interest Rate			12.00%
Principal Outstanding at Date of Acquisition			$ 8,410,000

[1] Previously reported bond purchase commitment that converted to a MRB in May 2018.