Fair Value Measurements Available for sale securities (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring/Nonrecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Fair Value Disclosure	$ 137,018,622	$ 103,973,434
Total gains (lossses) (realized/unrealized) [Abstract]
Gain (Loss) on Investments	680,444	0	0
Deconsolidation of VIE	0	1,861,051	7,589,901
Consolidation of VIEs			2,752,283
Proceeds from Maturities, Prepayments and Calls of Available-for-sale Securities	970,298	1,023,709	547,094
Assets, Fair Value Disclosure	243,126,815	137,018,622	103,973,434
Tax-exempt mortgage revenue bonds [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Assets, Fair Value Disclosure	145,237,376	135,695,352	100,566,643
Public housing capital fund trusts [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Assets, Fair Value Disclosure	65,389,298
Trust Portfolio--MBS Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Method Investment, Quoted Market Value	0
Total gains (lossses) (realized/unrealized) [Abstract]
Assets, Fair Value Disclosure	32,121,412
Fair Value Observable inputs (level 2)	32,121,412
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	0
Interest rate derivatives [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Assets, Fair Value Disclosure	378,729	1,323,270	3,406,791
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	137,018,622	103,973,434	69,540,270
Total gains (lossses) (realized/unrealized) [Abstract]
Gain (Loss) on Investments	(944,541)	(2,083,521)	571,684
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Income (Loss), before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	7,502,553	9,734,259	1,348,966
Deconsolidation of VIE		15,083,757	12,371,004
Consolidation of VIEs			(9,539,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	98,646,757	20,117,500	30,799,443
Proceeds from Maturities, Prepayments and Calls of Available-for-sale Securities	(30,654,939)	(9,526,619)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(563,049)	(280,188)	(1,118,933)
Assets, Fair Value Disclosure	211,005,403	137,018,622	103,973,434
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	211,005,403	137,018,622	103,973,434
Fair Value, Inputs, Level 3 [Member] | Tax-exempt mortgage revenue bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring/Nonrecurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Fair Value Disclosure	135,695,352	100,566,643	69,399,763
Total gains (lossses) (realized/unrealized) [Abstract]
Gain (Loss) on Investments	0	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	8,070,888	9,734,259	1,348,966
Deconsolidation of VIE		15,083,757	12,371,004
Consolidation of VIEs			(9,539,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	32,660,864	20,117,500	28,104,843
Proceeds from Maturities, Prepayments and Calls of Available-for-sale Securities	(30,654,939)	(9,526,619)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(534,789)	(280,188)	(1,118,933)
Assets, Fair Value Disclosure	145,237,376	135,695,352	100,566,643
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	145,237,376	135,695,352	100,566,643
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	0
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	(568,335)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	65,985,893
Proceeds from Maturities, Prepayments and Calls of Available-for-sale Securities	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(28,260)
Assets, Fair Value Disclosure	65,389,298
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	65,389,298
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	1,323,270	3,406,791	140,507
Total gains (lossses) (realized/unrealized) [Abstract]
Gain (Loss) on Investments	(944,541)	(2,083,521)	571,684
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	0
Deconsolidation of VIE		0	0
Consolidation of VIEs			0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0	2,694,600
Proceeds from Maturities, Prepayments and Calls of Available-for-sale Securities		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0	0
Assets, Fair Value Disclosure	378,729	1,323,270	3,406,791
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	378,729	1,323,270	3,406,791
Fair Value, Inputs, Level 1 [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Method Investment, Quoted Market Value	0	0	0
Fair Value, Inputs, Level 1 [Member] | Tax-exempt mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Method Investment, Quoted Market Value	0	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
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	0
Fair Value, Inputs, Level 2 [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Fair Value Observable inputs (level 2)	32,121,412	0	0
Fair Value, Inputs, Level 2 [Member] | Tax-exempt mortgage revenue bonds [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Fair Value Observable inputs (level 2)	0	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
Fair Value, Inputs, Level 2 [Member] | Interest rate derivatives [Member]
Total gains (lossses) (realized/unrealized) [Abstract]
Fair Value Observable inputs (level 2)	$ 0	$ 0	$ 0