Consolidated Statements of Comprehensive Income - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net income	$ 41,139,529	$ 30,662,851	$ 23,783,684
Reversal of net unrealized gain on sale of securities			(236,439)
Unrealized gain (loss) on bond purchase commitments	(3,002,540)	603,091	(3,234,911)
Comprehensive income	24,493,741	67,391,197	1,715,481
Comprehensive income (loss) allocated to noncontrolling interest		71,653	(823)
Partnership comprehensive income	24,493,741	67,319,544	1,716,304
Accumulated Other Comprehensive Income (Loss) [Member]
Net income	41,139,529	30,662,851	23,783,684
Reversal of net unrealized gain on sale of securities			(236,439)
Reversal of net unrealized losses (gains) on securities with other-than-temporary impairment	525,446	(672,097)
Unrealized gain (loss) on securities	(14,168,694)	36,797,352	(18,596,853)
Unrealized gain (loss) on bond purchase commitments	(3,002,540)	603,091	(3,234,911)
Commitments [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Unrealized gain (loss) on bond purchase commitments	$ (3,002,540)	$ 603,091	$ (3,234,911)