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, 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)	(216,948)			(2,169)		(214,779)
Distributions paid or accrued:
Regular distribution	(16,608,950)			(166,089)		(16,442,861)
Distribution of Tier 2 and Tier 3 income (Note 3)		$ (8,248,474)	$ (7,637,602)		$ (2,062,118)		$ (6,186,356)	$ (7,637,602)
Net income allocable to Partners	38,268,479			2,285,943		35,982,536
Sale of Beneficial Unit Certificates, net of issuance costs, value	1,953,829					1,953,829
Sale of Beneficial Unit Certificates, net of issuance costs, unit									349,136
Repurchase of BUCs, Value	(1,697,613)					(1,697,613)
Repurchase of BUCs, Units									(268,575)
Restricted units awarded									309,212
Restricted units compensation expense	1,822,525			18,225		1,804,300
Restricted units forfeited									(6,957)
BUCs surrendered to pay tax withholding on vested restricted units	(363,987)					(363,987)
BUCs surrendered to pay tax withholding, Units									(65,023)
Unrealized gain (loss) on securities	(14,168,694)			(141,687)		(14,027,007)				(14,168,694)
Unrealized loss on bond purchase commitments	(3,002,540)			(30,025)		(2,972,515)				(3,002,540)
Reversal of net unrealized loss on securities with other-than-temporary impairment	525,446			5,254		520,192				525,446
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 (Note 3)		$ (8,072,809)	$ (11,081,091)		$ (2,018,202)		$ (6,054,607)	$ (11,081,091)
Net income allocable to Partners	27,621,100			2,102,874		25,518,226
Restricted units awarded									353,197
Restricted units 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 (loss) 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