Consolidated Statement of Comprehensive Income - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net income	$ 30,662,851	$ 23,783,684	$ 26,606,222
Reversal of net unrealized gain on sale of securities		(236,439)
Reversal of net unrealized gain on securities with other-than-temporary impairment	(672,097)
Unrealized gain (loss) on bond purchase commitments	603,091	(3,234,911)	(146,053)
Comprehensive income	67,391,197	1,715,481	36,502,410
Comprehensive income (loss) allocated to noncontrolling interest	71,653	(823)	(2,801)
Partnership comprehensive income	67,319,544	1,716,304	36,505,211
Accumulated Other Comprehensive Income (Loss) [Member]
Net income	30,662,851	23,783,684	26,606,222
Reversal of net unrealized gain on sale of securities		(236,439)
Reversal of net unrealized gain on securities with other-than-temporary impairment	(672,097)
Unrealized gain (loss) on securities	36,797,352	(18,596,853)	10,042,241
Unrealized gain (loss) on bond purchase commitments	603,091	(3,234,911)	(146,053)
Commitments [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Unrealized gain (loss) on bond purchase commitments	$ 603,091	$ (3,234,911)	$ (146,053)