Investments in Mortgage Revenue Bonds - Schedule of MRBs Redeemed (Details)	9 Months Ended
	Sep. 30, 2019 USD ($) Unit	Sep. 30, 2018 USD ($) Unit
Schedule Of Available For Sale Securities [Line Items]
Units | Unit	2,886
Mortgage Revenue Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Principal Outstanding at Date of Redemption | $	$ 11,802,000	$ 39,014,000
Mortgage Revenue Bonds [Member] | Seasons San Juan Capistrano [Member] | Series B [Member] | San Juan Capistrano California [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Redeemed	January
Units | Unit	112
Original Maturity Date	Jan. 01, 2019
Base Interest Rate	8.00%
Principal Outstanding at Date of Redemption | $	$ 5,574,000
Mortgage Revenue Bonds [Member] | Courtyard Apartments [Member] | Series B [Member] | Fullerton Califonia [Member]
Schedule Of Available For Sale Securities [Line Items]
Month Redeemed	April
Units | Unit	108
Original Maturity Date	Jun. 01, 2019
Base Interest Rate	8.00%
Principal Outstanding at Date of Redemption | $	$ 6,228,000
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 [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 [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 Beach, 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