Marketable Securities - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 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	23,106,000		23,106,000		$ 34,874,000
Gross realized gain related to the sale of marketable securities	35,000		35,000
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)			26 years
Settlement basis period (in days)			7 days
Effective maturity period (in days)			7 days