Cash and Investments in Marketable Securities - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Cash and Investments in Marketable Securities [Line Items]
Maximum maturity term for debt securities investment			2 years
Weighted average maturity term for debt securities investment (or less)			1 year
Available-for-sale securities, sold	$ 41,700,000	$ 0	$ 41,700,000	$ 0
Accrued interest receivable	$ 6,100,000		$ 6,100,000		$ 6,500,000
Minimum
Cash and Investments in Marketable Securities [Line Items]
Long term investment maturity period			1 year		1 year
Maximum
Cash and Investments in Marketable Securities [Line Items]
Long term investment maturity period			2 years		2 years