Condensed Consolidated Statements of Partners' Capital (Unaudited) - USD ($)	Total	Tier 2 [Member]	General Partner [Member]	General Partner [Member] Tier 2 [Member]	Beneficial Unit Certificate Holders - Restricted and Unrestricted [Member]	Beneficial Unit Certificate Holders - Restricted and Unrestricted [Member] Tier 2 [Member]	Number of Units - Restricted and Unrestricted [Member]	Non-controlling Interest [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
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	(11,819,553)		(118,196)		(11,701,357)
Distribution of Tier 2 earnings (Note 3)		$ (4,482,500)		$ (1,120,625)		$ (3,361,875)
Net income allocable to Partners	10,712,723		1,182,211		9,458,859			$ 71,653
Repurchase of Beneficial Unit Certificates	(1,466,222)				(1,466,222)
Repurchase of Beneficial Unit Certificates, Units							(254,656)
Restricted units awarded							283,046
Restricted units compensation expense	609,733		6,097		603,636
Unrealized gain (loss) on securities	29,207,054		292,071		28,914,983				29,207,054
Unrealized gain (loss) on bond purchase commitments	765,723		7,657		758,066				765,723
Balance at Jun. 30, 2017	303,584,510		351,751		303,232,759				68,868,261
Partners' Capital Account, Units at Jun. 30, 2017							60,252,928
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
Regular distribution	(15,265,891)		(152,659)		(15,113,232)
Net income allocable to Partners	7,906,900		79,069		7,827,831
Sale of Beneficial Unit Certificates, net of issuance costs	192,310				192,310
Sale of Beneficial Unit Certificates, net of issuance costs, Units							38,617
Repurchase of Beneficial Unit Certificates	(1,697,613)				(1,697,613)
Repurchase of Beneficial Unit Certificates, Units							(268,575)
Restricted units awarded							309,212
Restricted units compensation expense	750,157		7,502		742,655
Unrealized gain (loss) on securities	(17,353,309)		(173,533)		(17,179,776)				(17,353,309)
Unrealized gain (loss) on bond purchase commitments	(2,007,855)		(20,079)		(1,987,776)				(2,007,855)
Reversal of net unrealized loss on securities with other-than-temporary impairment	525,446		5,254		520,192				525,446
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