Investments in Mortgage Revenue Bonds ("MRBs") - Schedule of MRBs Redeemed (Details)	9 Months Ended
	Sep. 30, 2018 USD ($) Unit	Sep. 30, 2017 USD ($) Unit
Schedule Of Available For Sale Securities [Line Items]
Principal Outstanding at Date of Redemption		$ 1,997,000
Harmony Court Bakersfield [Member] | Series B [Member] | Bakersfield, California [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Redeemed		August
Units | Unit		96
Original Maturity Date		Dec. 01, 2018
Base Interest Rate		5.50%
Principal Outstanding at Date of Redemption		$ 1,997,000
Mortgage Revenue Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Principal Outstanding at Date of Redemption	$ 39,014,000
Mortgage Revenue Bonds [Member] | Sycamore Walk [Member] | Series B [Member] | Bakersfield, California [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Redeemed	January
Units | Unit	112
Original Maturity Date	Jan. 01, 2018
Base Interest Rate	8.00%
Principal Outstanding at Date of Redemption	$ 1,815,000
Mortgage Revenue Bonds [Member] | Seasons Lakewood [Member] | Series B [Member] | Lakewood, California [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Redeemed	March
Units | Unit	85
Original Maturity Date	Jan. 01, 2019
Base Interest Rate	8.00%
Principal Outstanding at Date of Redemption	$ 5,260,000
Mortgage Revenue Bonds [Member] | Summerhill [Member] | Series B [Member] | Bakersfield, California [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Redeemed	March
Units | Unit	128
Original Maturity Date	Dec. 01, 2018
Base Interest Rate	8.00%
Principal Outstanding at Date of Redemption	$ 3,372,000
Mortgage Revenue Bonds [Member] | Oaks at Georgetown [Member] | Series B [Member] | Georgetown, Texas [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Redeemed	April
Units | Unit	192
Original Maturity Date	Jan. 01, 2019
Base Interest Rate	8.00%
Principal Outstanding at Date of Redemption	$ 5,512,000
Mortgage Revenue Bonds [Member] | Seasons at Simi Valley [Member] | Series B [Member] | Simi Valley, California [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Redeemed	April
Units | Unit	69
Original Maturity Date	Sep. 01, 2018
Base Interest Rate	8.00%
Principal Outstanding at Date of Redemption	$ 1,944,000
Mortgage Revenue Bonds [Member] | San Vicente [Member] | Series B [Member] | Soledad, California [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Redeemed	May
Units | Unit	50
Original Maturity Date	Nov. 01, 2018
Base Interest Rate	8.00%
Principal Outstanding at Date of Redemption	$ 1,825,000
Mortgage Revenue Bonds [Member] | The Village at Madera [Member] | Series B [Member] | Madera, California [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Redeemed	May
Units | Unit	75
Original Maturity Date	Dec. 01, 2018
Base Interest Rate	8.00%
Principal Outstanding at Date of Redemption	$ 1,719,000
Mortgage Revenue Bonds [Member] | Las Palmas Series B [Member] | Series B [Member] | Coachella, California [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Redeemed	July
Units | Unit	81
Original Maturity Date	Nov. 01, 2018
Base Interest Rate	8.00%
Principal Outstanding at Date of Redemption	$ 1,770,000
Mortgage Revenue Bonds [Member] | Harmony Terrace - Series B [Member] | Series B [Member] | Simi Valley, California [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Redeemed	August
Units | Unit	136
Original Maturity Date	Jan. 01, 2019
Base Interest Rate	8.00%
Principal Outstanding at Date of Redemption	$ 7,400,000
Mortgage Revenue Bonds [Member] | Lake Forest [Member] | Daytona Florida [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Redeemed	September
Units | Unit	240
Original Maturity Date	Dec. 01, 2031
Base Interest Rate	6.25%
Principal Outstanding at Date of Redemption	$ 8,397,000