Marketable Securities - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Marketable Securities [Abstract]
Fair value of securities owned	$ 300,000		$ 300,000		$ 14,200,000
Realized and unrealized net gains on marketable securities	$ 25,600	$ 400,000	300,000	$ 5,700,000
Marketable securities sold at fair value			14,200,000	24,600,000
Purchase of marketable securities			$ 0	$ 0