Marketable Securities - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Marketable securities	$ 494,200,000		$ 395,100,000
Marketable securities unrealized loss, Less than twelve months	473,700,000		280,300,000
Marketable securities unrealized loss, more than twelve months	0		$ 0
Unrealized loss, net	400,000	$ 0
Available-for-sale debt securities remaining maturities greater than two years	$ 0