Consolidated Statements of Partners' Capital - USD ($)	Total	Tier 2 [Member]	Tier 3 [Member]	General Partner [Member]	General Partner [Member] Tier 2 [Member]	BUCs - Restricted and Unrestricted [Member]	BUCs - Restricted and Unrestricted [Member] Tier 2 [Member]	BUCs - Restricted and Unrestricted [Member] Tier 3 [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 2)	(212)			(2)		(210)
Distributions paid or accrued:
Regular distribution	(13,379,769)			(133,799)		(13,245,970)
Distribution of Tier 2 and Tier 3 (income) loss (Note 3)		$ (8,072,809)	$ (11,081,091)		$ (2,018,202)		$ (6,054,607)	$ (11,081,091)
Net income (loss) allocable to Partners	27,621,100			2,102,874		25,518,226
Restricted units awarded									353,197
Restricted unit compensation expense	3,636,091			36,361		3,599,730
BUCs surrendered to pay tax withholding on vested restricted units	(1,581,423)					(1,581,423)
BUCs surrendered to pay tax withholding, Units									(209,460)
Unrealized gain on securities	40,330,635			403,306		39,927,329				40,330,635
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	(19,110,367)			(191,104)		(18,919,263)
Distribution of Tier 2 and Tier 3 (income) loss (Note 3)		$ 445,719			$ 80,501		$ 365,218
Net income (loss) allocable to Partners	4,337,777			(32,666)		4,370,443
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	1,017,938			10,179		1,007,759
Restricted units forfeited									(2,802)
BUCs surrendered to pay tax withholding on vested restricted units	(35,945)					(35,945)
BUCs surrendered to pay tax withholding, Units									(8,728)
Unrealized gain on securities	33,609,375			336,094		33,273,281				33,609,375
Unrealized gain on bond purchase commitments	431,879			4,319		427,560				431,879
Reversal of net unrealized gains onsale 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	652,880			6,529		646,351				652,880
Balance at Dec. 31, 2020	$ 359,772,042			$ 934,892		$ 358,837,150				$ 132,594,007
Partners' Capital Account, Units at Dec. 31, 2020									60,823,674