Marketable Securities - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for a period longer than twelve months	$ 0		$ 0
Allowance for credit losses	0		0
Impairment charges	0	$ 0	0	$ 0
Marketable securities, due within one year	20,457,000		20,457,000		$ 34,874,000
Gross realized gain related to the sale of marketable securities	0	$ 0	35,000	$ 0
Variable rate demand notes
Debt Securities, Available-for-sale [Line Items]
Marketable securities, due within one year	$ 5,600,000		$ 5,600,000
Variable rate demand notes | Minimum
Debt Securities, Available-for-sale [Line Items]
Contractual maturities (in years)			17 years
Settlement basis period (in days)			1 day
Effective maturity period (in days)			1 day
Variable rate demand notes | Maximum
Debt Securities, Available-for-sale [Line Items]
Contractual maturities (in years)			25 years
Settlement basis period (in days)			7 days
Effective maturity period (in days)			7 days