Marketable Securities - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) Security	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) Security	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Available for sale debt securities remaining maturities greater than three years	$ 0		$ 0		$ 0
Number of investment grade fixed income debt security | Security	47		47
Allowance for credit loss	$ 0		$ 0		$ 0
Other-than-temporary impaired	$ 0	$ 0	$ 0	$ 0
Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Marketable securities, unrealized gain (loss)		7,500,000		8,300,000
Marketable securities, realized gain (loss)		$ 7,500,000		$ 8,300,000