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]	Noncontrolling Interest [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance at Dec. 31, 2015	$ 313,124,827			$ 399,077		$ 312,720,264				$ 5,486	$ 60,963,687
Partners' Capital Account, Units at Dec. 31, 2015									60,252,928
Reversal of net unrealized gain on sale of securities	(236,439)			(2,364)		(234,075)					(236,439)
Distributions paid or accrued:
Regular distribution	(21,764,612)			(217,646)		(21,546,966)
Distribution of Tier 2 and Tier 3 income (Note 3)		$ (11,434,599)			$ (2,858,650)		$ (8,575,949)
Net income (loss) allocable to Partners	23,200,277			2,992,106		20,208,994				(823)
Repurchase of BUCs	(1,603,658)					(1,603,658)
Repurchase of BUCs									(272,307)
Restricted units awarded									272,307
Restricted units compensation expense	833,142			8,331		824,811
BUCs surrendered to pay tax withholding on vested restricted units	(153,306)					(153,306)
BUCs surrendered to pay tax withholding, Units									(28,390)
Unrealized gain (loss) on securities	(18,596,853)			(185,969)		(18,410,884)					(18,596,853)
Unrealized gain (loss) on bond purchase commitment	(3,234,911)			(32,349)		(3,202,562)					(3,234,911)
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	(19,427,246)			(194,272)		(19,232,974)
Distribution of Tier 2 and Tier 3 income (Note 3)		(7,978,073)	$ (4,928,231)		(1,994,518)		(5,983,555)	$ (4,928,231)
Net income (loss) allocable to Partners	28,680,313			2,140,074		26,468,586				$ 71,653
Sale of Beneficial Unit Certificates, net of issuance costs, value	805,890					805,890
Sale of Beneficial Unit Certificates, net of issuance costs, unit									161,383
Repurchase of BUCs	(1,466,222)					(1,466,222)
Repurchase of BUCs									(254,656)
Restricted units awarded									283,046
Restricted units compensation expense	1,615,242			16,152		1,599,090
BUCs surrendered to pay tax withholding on vested restricted units	(247,301)					(247,301)
BUCs surrendered to pay tax withholding, Units									(40,637)
Unrealized gain (loss) on securities	36,797,352			367,974		36,429,378					36,797,352
Unrealized gain (loss) on bond purchase commitment	603,091			6,031		597,060					603,091
Reversal of net unrealized gain on securities with other-than-temporary impairment	(672,097)			(6,721)		(665,376)					(672,097)
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 (loss) 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	(1,697,613)					(1,697,613)
Repurchase of BUCs									(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 gain (loss) on bond purchase commitment	(3,002,540)			(30,025)		(2,972,515)					(3,002,540)
Reversal of net unrealized gain 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