Marketable Securities (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Marketable Securities [Abstract]
Fair value of securities owned	$ 300		$ 300		$ 300
Realized and unrealized net gains (loss) on marketable securities	$ (12)	$ 26	(1)	$ 300
Marketable securities sold at fair value during period			0	14,200
Purchase of marketable securities			$ 0	$ 0