Investments - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Marketable Securities [Abstract]
Impairment of investments	$ 0	$ 0	$ 0	$ 0
Ownership interest in four limited partnerships, fair value	4,244,663		4,244,663		$ 4,051,399
Ownership interest in four limited partnerships, cost	$ 3,550,000		$ 3,550,000		$ 3,547,687