Condensed Consolidated Statements of Partners' Capital (Unaudited) - USD ($)	Total	Tier 2 [Member]	General Partner [Member]	General Partner [Member] Tier 2 [Member]	Beneficial Unit Certificate Holders - Restricted and Unrestricted [Member]	Beneficial Unit Certificate Holders - Restricted and Unrestricted [Member] Tier 2 [Member]	Number of Units - Restricted and Unrestricted [Member]	Non-controlling Interest [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance at Dec. 31, 2016	$ 280,133,868		$ 102,536		$ 280,026,669			$ 4,663	$ 38,895,484
Partners' Capital Account, Units at Dec. 31, 2016							60,224,538
Distribution to noncontrolling interest	(76,316)							(76,316)
Distributions paid or accrued
Regular distribution	(4,261,023)		(42,610)		(4,218,413)
Distribution of Tier 2 earnings (Note 3)		$ (4,417,604)		$ (1,104,401)		$ (3,313,203)
Net income allocable to Partners	7,035,873		1,147,072		5,817,148			$ 71,653
Repurchase of Beneficial Unit Certificates	(823,358)				(823,358)
Repurchase of Beneficial Unit Certificates, Units							(144,748)
Restricted units awarded							173,138
Restricted units compensation expense	170,840		1,708		169,132
Unrealized gain on securities	18,980,366		189,804		18,790,562				18,980,366
Unrealized gain (loss) on bond purchase commitments	220,944		2,209		218,735				220,944
Balance at Mar. 31, 2017	296,963,590		296,318		296,667,272				58,096,794
Partners' Capital Account, Units at Mar. 31, 2017							60,252,928
Balance at Dec. 31, 2017	313,840,270		437,256		313,403,014				75,623,830
Partners' Capital Account, Units at Dec. 31, 2017							60,373,674
Cumulative effect of accounting change (Note 2) at Dec. 31, 2017	(216,948)		(2,169)		(214,779)
Distributions paid or accrued
Regular distribution	(7,632,945)		(76,329)		(7,556,616)
Net income allocable to Partners	5,286,541		52,865		5,233,676
Sale of Beneficial Unit Certificates, net of issuance costs	192,310				192,310
Sale of Beneficial Unit Certificates, net of issuance costs, Units							38,617
Repurchase of Beneficial Unit Certificates	(1,256,654)				(1,256,654)
Repurchase of Beneficial Unit Certificates, Units							(198,465)
Restricted units awarded							239,102
Restricted units compensation expense	206,636		2,066		204,570
Unrealized gain on securities	(21,874,876)		(218,749)		(21,656,127)				(21,874,876)
Unrealized gain (loss) on bond purchase commitments	(975,067)		(9,751)		(965,316)				(975,067)
Balance at Mar. 31, 2018	$ 287,569,267		$ 185,189		$ 287,384,078				$ 52,773,887
Partners' Capital Account, Units at Mar. 31, 2018							60,452,928