Fair Value Measurements Available for sale securities (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring/Nonrecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Fair Value Disclosure			$ 535,793,465
Total gains (lossses) (realized/unrealized) [Abstract]
Mortgage revenue bonds, at fair value	81,875,851		70,601,045
Assets, Fair Value Disclosure	592,028,676		535,793,465
Mortgage revenue bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring/Nonrecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Fair Value Disclosure			449,024,137
Total gains (lossses) (realized/unrealized) [Abstract]
Assets, Fair Value Disclosure	507,898,698		449,024,137
Bond Purchase Commitment [Member]
Fair Value, Assets and Liabilities Measured on Recurring/Nonrecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Fair Value Disclosure			5,780,413
Total gains (lossses) (realized/unrealized) [Abstract]
Assets, Fair Value Disclosure	5,204,188		5,780,413
Public housing capital fund trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring/Nonrecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Fair Value Disclosure			61,263,123
Total gains (lossses) (realized/unrealized) [Abstract]
Assets, Fair Value Disclosure	60,272,941		61,263,123
Trust Portfolio--MBS Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring/Nonrecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Fair Value Disclosure			14,841,558
Total gains (lossses) (realized/unrealized) [Abstract]
Assets, Fair Value Disclosure	14,884,339		14,841,558
Taxable Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring/Nonrecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Fair Value Disclosure			4,616,565
Total gains (lossses) (realized/unrealized) [Abstract]
Assets, Fair Value Disclosure	4,411,214		4,616,565
Interest rate derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring/Nonrecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Fair Value Disclosure			267,669
Total gains (lossses) (realized/unrealized) [Abstract]
Assets, Fair Value Disclosure	(642,704)		267,669
Fair Value, Inputs, Level 1 [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Method Investment, Quoted Market Value	0		0
Fair Value, Inputs, Level 1 [Member] | Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Method Investment, Quoted Market Value	0		0
Fair Value, Inputs, Level 1 [Member] | Bond Purchase Commitment [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Method Investment, Quoted Market Value	0		0
Fair Value, Inputs, Level 1 [Member] | Public housing capital fund trusts [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Method Investment, Quoted Market Value	0		0
Fair Value, Inputs, Level 1 [Member] | Trust Portfolio--MBS Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Method Investment, Quoted Market Value	0		0
Fair Value, Inputs, Level 1 [Member] | Taxable Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Method Investment, Quoted Market Value	0		0
Fair Value, Inputs, Level 1 [Member] | Interest rate derivatives [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Method Investment, Quoted Market Value	0		0
Fair Value, Inputs, Level 2 [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Fair Value Observable inputs (level 2)	14,884,339		14,841,558
Fair Value, Inputs, Level 2 [Member] | Mortgage revenue bonds [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Fair Value Observable inputs (level 2)	0		0
Fair Value, Inputs, Level 2 [Member] | Bond Purchase Commitment [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Fair Value Observable inputs (level 2)	0		0
Fair Value, Inputs, Level 2 [Member] | Public housing capital fund trusts [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Fair Value Observable inputs (level 2)	0		0
Fair Value, Inputs, Level 2 [Member] | Trust Portfolio--MBS Bonds [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Fair Value Observable inputs (level 2)	14,884,339		14,841,558
Fair Value, Inputs, Level 2 [Member] | Taxable Bonds [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Fair Value Observable inputs (level 2)	0		0
Fair Value, Inputs, Level 2 [Member] | Interest rate derivatives [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Fair Value Observable inputs (level 2)	0		0
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring/Nonrecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Fair Value Disclosure	520,951,907	352,338,623
Total gains (lossses) (realized/unrealized) [Abstract]
Gain (Loss) on Investments	(899,873)	(175,837)
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Income (Loss), before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	(1,683,508)	15,953,883
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	58,945,000
Deriviative at purchase price	(10,500)	391,500
Assets, Fair Value Disclosure	577,144,337	383,945,007
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(158,689)	(1,705,824)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	577,144,337		520,951,907
Fair Value, Inputs, Level 3 [Member] | Mortgage revenue bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring/Nonrecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Fair Value Disclosure	449,024,137	285,318,171
Total gains (lossses) (realized/unrealized) [Abstract]
Gain (Loss) on Investments	0	0
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Income (Loss), before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	74,600	14,073,020
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	58,945,000
Deriviative at purchase price	0	0
Assets, Fair Value Disclosure	507,898,698	316,471,727
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(145,039)	(62,126)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	507,898,698		449,024,137
Fair Value, Inputs, Level 3 [Member] | Bond Purchase Commitment [Member]
Fair Value, Assets and Liabilities Measured on Recurring/Nonrecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Fair Value Disclosure	5,780,413
Total gains (lossses) (realized/unrealized) [Abstract]
Gain (Loss) on Investments	0
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Income (Loss), before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	(576,225)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0
Deriviative at purchase price	0
Assets, Fair Value Disclosure	5,204,188
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	5,204,188		5,780,413
Fair Value, Inputs, Level 3 [Member] | Public housing capital fund trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring/Nonrecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Fair Value Disclosure	61,263,123	62,056,379
Total gains (lossses) (realized/unrealized) [Abstract]
Gain (Loss) on Investments	0	0
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Income (Loss), before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	(976,532)	1,657,859
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0
Deriviative at purchase price	0	0
Assets, Fair Value Disclosure	60,272,941	62,070,540
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(13,650)	(1,643,698)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	60,272,941		61,263,123
Fair Value, Inputs, Level 3 [Member] | Trust Portfolio--MBS Bonds [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	0		0
Fair Value, Inputs, Level 3 [Member] | Taxable Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring/Nonrecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Fair Value Disclosure	4,616,565	4,075,953
Total gains (lossses) (realized/unrealized) [Abstract]
Gain (Loss) on Investments	0	0
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Income (Loss), before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	(205,351)	223,004
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0
Deriviative at purchase price	0	0
Assets, Fair Value Disclosure	4,411,214	4,298,957
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	4,411,214		4,616,565
Fair Value, Inputs, Level 3 [Member] | Interest rate derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring/Nonrecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Fair Value Disclosure	267,669	888,120
Total gains (lossses) (realized/unrealized) [Abstract]
Gain (Loss) on Investments	(899,873)	(175,837)
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Income (Loss), before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0
Deriviative at purchase price	(10,500)	391,500
Assets, Fair Value Disclosure	(642,704)	1,103,783
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	(642,704)		267,669
Greens of Pine Glen [Member] | Fair Value, Inputs, Level 3 [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		34,778,800
Greens of Pine Glen [Member] | Fair Value, Inputs, Level 3 [Member] | Mortgage revenue bonds [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		34,778,800
Greens of Pine Glen [Member] | Fair Value, Inputs, Level 3 [Member] | Public housing capital fund trusts [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0
Greens of Pine Glen [Member] | Fair Value, Inputs, Level 3 [Member] | Taxable Bonds [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0
Greens of Pine Glen [Member] | Fair Value, Inputs, Level 3 [Member] | Interest rate derivatives [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0
Lost Creek [Member] | Fair Value, Inputs, Level 3 [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Mortgage revenue bonds, at fair value		(17,636,138)
Lost Creek [Member] | Fair Value, Inputs, Level 3 [Member] | Mortgage revenue bonds [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Mortgage revenue bonds, at fair value		(17,636,138)
Lost Creek [Member] | Fair Value, Inputs, Level 3 [Member] | Public housing capital fund trusts [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Mortgage revenue bonds, at fair value		0
Lost Creek [Member] | Fair Value, Inputs, Level 3 [Member] | Taxable Bonds [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Mortgage revenue bonds, at fair value		0
Lost Creek [Member] | Fair Value, Inputs, Level 3 [Member] | Interest rate derivatives [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Mortgage revenue bonds, at fair value		$ 0