Condensed Consolidated Statements of Partners' Capital (Unaudited) - USD ($)	Total	Tier 2 [Member]	General Partner [Member]	General Partner [Member] Tier 2 [Member]	BUCs - Restricted and Unrestricted [Member]	BUCs - Restricted and Unrestricted [Member] Tier 2 [Member]	Number of BUCs - Restricted and Unrestricted [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance at Dec. 31, 2018	$ 304,465,741		$ 344,590		$ 304,121,151			$ 58,978,042
Partners' Capital Account, Units at Dec. 31, 2018							60,691,467
Cumulative effect of accounting change (Note 13)	(212)		(2)		(210)
Distributions paid or accrued:
Regular distribution	(5,381,169)		(53,812)		(5,327,357)
Distribution of Tier 2 loss (income) (Note 3)		$ (3,012,102)		$ (753,025)		$ (2,259,077)
Net income (loss) allocable to Partners	5,734,050		780,245		4,953,805
Restricted unit compensation expense	184,184		1,842		182,342
Unrealized gain (loss) on securities	8,143,927		81,439		8,062,488			8,143,927
Balance at Mar. 31, 2019	310,134,419		401,277		309,733,142			67,121,969
Partners' Capital Account, Units at Mar. 31, 2019							60,691,467
Balance at Dec. 31, 2018	304,465,741		344,590		304,121,151			58,978,042
Partners' Capital Account, Units at Dec. 31, 2018							60,691,467
Distributions paid or accrued:
Net income (loss) allocable to Partners	8,902,477
Balance at Jun. 30, 2019	320,746,093		507,393		320,238,700			82,042,050
Partners' Capital Account, Units at Jun. 30, 2019							60,691,467
Balance at Mar. 31, 2019	310,134,419		401,277		309,733,142			67,121,969
Partners' Capital Account, Units at Mar. 31, 2019							60,691,467
Distributions paid or accrued:
Regular distribution	(7,663,064)		(76,631)		(7,586,433)
Net income (loss) allocable to Partners	3,168,427		31,684		3,136,743
Restricted unit compensation expense	186,230		1,862		184,368
Unrealized gain (loss) on securities	14,920,081		149,201		14,770,880			14,920,081
Balance at Jun. 30, 2019	320,746,093		507,393		320,238,700			82,042,050
Partners' Capital Account, Units at Jun. 30, 2019							60,691,467
Balance at Dec. 31, 2019	341,938,263		735,128		341,203,135			99,308,677
Partners' Capital Account, Units at Dec. 31, 2019							60,835,204
Distributions paid or accrued:
Regular distribution	(8,050,119)		(80,501)		(7,969,618)
Distribution of Tier 2 loss (income) (Note 3)		$ 445,719		$ 80,501		$ 365,218
Net income (loss) allocable to Partners	2,263,994		(53,404)		2,317,398
Repurchase of BUCs, Value	(2,106,673)				(2,106,673)
Repurchase of BUCs, Units							(290,000)
Restricted units awarded							290,000
Restricted unit compensation expense	39,068		391		38,677
Unrealized gain (loss) on securities	(7,057,736)		(70,577)		(6,987,159)			(7,057,736)
Reversal of net unrealized gains on sale of securities	(1,408,804)		(14,088)		(1,394,716)			(1,408,804)
Reversal of net unrealized loss on securities to provision for credit loss	372,169		3,722		368,447			372,169
Balance at Mar. 31, 2020	326,435,881		601,172		325,834,709			91,214,306
Partners' Capital Account, Units at Mar. 31, 2020							60,835,204
Balance at Dec. 31, 2019	341,938,263		735,128		341,203,135			99,308,677
Partners' Capital Account, Units at Dec. 31, 2019							60,835,204
Distributions paid or accrued:
Net income (loss) allocable to Partners	6,134,580
Reversal of net unrealized loss on securities to provision for credit loss								372,169
Balance at Jun. 30, 2020	347,887,402		815,686		347,071,716			112,185,955
Partners' Capital Account, Units at Jun. 30, 2020							60,835,204
Balance at Mar. 31, 2020	326,435,881		601,172		325,834,709			91,214,306
Partners' Capital Account, Units at Mar. 31, 2020							60,835,204
Distributions paid or accrued:
Regular distribution	(3,686,982)		(36,870)		(3,650,112)
Net income (loss) allocable to Partners	3,870,586		38,706		3,831,880
Restricted unit compensation expense	296,268		2,962		293,306
Unrealized gain (loss) on securities	20,971,649		209,716		20,761,933			20,971,649
Balance at Jun. 30, 2020	$ 347,887,402		$ 815,686		$ 347,071,716			$ 112,185,955
Partners' Capital Account, Units at Jun. 30, 2020							60,835,204