Marketable Securities - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for a period longer than twelve months	$ 17,600,000
Other-than-temporary impairment charges	0	$ 0
Marketable securities:	36,486,000		$ 37,121,000
Proceeds from maturities of available-for-sale securities	12,910,000	0
Proceeds from sale of marketable securities	0	$ 0
Variable rate demand notes
Debt Securities, Available-for-sale [Line Items]
Marketable securities:	$ 1,600,000
Variable rate demand notes | Minimum
Debt Securities, Available-for-sale [Line Items]
Contractual maturities (in years)	26 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