Condensed Consolidated Statements of Partners' Capital (Unaudited) - USD ($)	Total	Tier 3 [Member]	Tier 2 [Member]	General Partner [Member]	General Partner [Member] Tier 2 [Member]	BUCs - Restricted and Unrestricted [Member]	BUCs - Restricted and Unrestricted [Member] Tier 3 [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, 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 ($0.125 per BUC):
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, value	192,310					192,310
Sale of Beneficial Unit Certificates, net of issuance costs, unit									38,617
Repurchase of BUCs, Value	(1,256,654)					(1,256,654)
Repurchase of BUCs, Units									(198,465)
Restricted units awarded									239,102
Restricted unit compensation expense	206,636			2,066		204,570
Unrealized gain (loss) 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
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 ($0.125 per BUC):
Net income allocable to Partners	25,072,192
Balance at Sep. 30, 2018	288,499,259			195,059		288,304,200				50,095,363
Partners' Capital Account, Units at Sep. 30, 2018									60,520,261
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
Distributions paid or accrued ($0.125 per BUC):
Regular distribution	(7,632,946)			(76,330)		(7,556,616)
Net income allocable to Partners	2,620,359			26,204		2,594,155
Repurchase of BUCs, Value	(440,959)					(440,959)
Repurchase of BUCs, Units									(70,110)
Restricted units awarded									70,110
Restricted unit compensation expense	543,521			5,436		538,085
Unrealized gain (loss) on securities	4,521,567			45,216		4,476,351				4,065,221
Unrealized gain (loss) on bond purchase commitments	(1,032,788)			(10,328)		(1,022,460)				(1,032,788)
Reversal of Net Unrealized Loss on Securities with Other Than Temporary Impairment	525,446			5,254		520,192				981,792
Balance at Jun. 30, 2018	286,673,467			180,641		286,492,826				56,788,112
Partners' Capital Account, Units at Jun. 30, 2018									60,452,928
Distributions paid or accrued ($0.125 per BUC):
Regular distribution	(1,355,445)			(13,554)		(1,341,891)
Distribution of Tier 2 and Tier 3 income (Note 3)			$ (8,297,523)		$ (2,074,381)			$ (6,223,142)
Net income allocable to Partners	17,165,292			2,163,058		15,002,234
Sale of Beneficial Unit Certificates, net of issuance costs, value	383,990					383,990
Sale of Beneficial Unit Certificates, net of issuance costs, unit									67,333
Restricted unit compensation expense	622,227			6,222		616,005
Unrealized gain (loss) on securities	(6,744,509)			(67,445)		(6,677,064)				(6,288,163)
Unrealized gain (loss) on bond purchase commitments	51,760			518		51,242				51,760
Reversal of Net Unrealized Loss on Securities with Other Than Temporary Impairment										(456,346)
Balance at Sep. 30, 2018	288,499,259			195,059		288,304,200				50,095,363
Partners' Capital Account, Units at Sep. 30, 2018									60,520,261
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) at Dec. 31, 2018	(212)			(2)		(210)
Distributions paid or accrued ($0.125 per BUC):
Regular distribution	(5,381,169)			(53,812)		(5,327,357)
Distribution of Tier 2 and Tier 3 income (Note 3)			(3,012,102)		(753,025)			(2,259,077)
Net income 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
Cumulative effect of accounting change (Note 2) at Dec. 31, 2018	(212)			(2)		(210)
Distributions paid or accrued ($0.125 per BUC):
Net income allocable to Partners	17,892,618
Balance at Sep. 30, 2019	341,599,455			731,741		340,867,714				101,090,366
Partners' Capital Account, Units at Sep. 30, 2019									60,835,204
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 ($0.125 per BUC):
Regular distribution	(7,663,064)			(76,631)		(7,586,433)
Net income 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
Distributions paid or accrued ($0.125 per BUC):
Distribution of Tier 2 and Tier 3 income (Note 3)		$ (3,809,553)	$ (5,059,796)		$ (1,264,949)		$ (3,809,553)	$ (3,794,847)
Net income allocable to Partners	8,990,141			1,266,157		7,723,984
Restricted units awarded									353,197
Restricted unit compensation expense	3,265,677			32,657		3,233,020
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	19,048,316			190,483		18,857,833				19,048,316
Balance at Sep. 30, 2019	$ 341,599,455			$ 731,741		$ 340,867,714				$ 101,090,366
Partners' Capital Account, Units at Sep. 30, 2019									60,835,204