Marketable Securities - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net Gain (Loss) [Member]
Schedule of Available-for-sale Securities [Line Items]
Net gain (loss) recognized, mutual fund equity investment	$ 0.3	$ 0.0	$ 0.4	$ 0.0
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale short-term investments period of contractual maturities			1 year
Available-for-sale long-term investments period of contractual maturities			2 years
Minimum [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale long-term investments period of contractual maturities			1 year