Marketable Securities - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for a period longer than twelve months	$ 9,200,000		$ 9,200,000
Other-than-temporary impairment charges	0	$ 0	0	$ 0
Marketable securities, due within one year	39,288,000		39,288,000		$ 37,121,000
Proceeds from maturities of available-for-sale securities	9,700,000		22,560,000	0
Proceeds from sale of marketable securities	0	$ 0	0	$ 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)			18 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)			27 years
Settlement basis period (in days)			7 days
Effective maturity period (in days)			7 days