Marketable Securities (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Marketable Securities [Abstract]
Fair value of securities owned	$ 0.4	$ 0.3
Realized and unrealized net gains on marketable securities	0.1	0.4	$ 6.7
Marketable securities sold at fair value		14.2
Purchase of marketable securities	$ 0.0	$ 0.0