Marketable Securities - Available-For-Sale Securities (Details) - USD ($)	Jun. 30, 2023	Dec. 31, 2022
Available-for-sale securities:
Amortized Cost	$ 18,040,000	$ 22,992,000
Gross Unrealized Gain	1,000	2,000
Gross Unrealized (Losses)	(517,000)	(715,000)
Estimated Fair Value	17,524,000	22,279,000
Held-to-maturity securities:
Amortized Cost		0
Tax-exempt municipal securities
Available-for-sale securities:
Amortized Cost	14,835,000	15,852,000
Gross Unrealized Gain	1,000	2,000
Gross Unrealized (Losses)	(365,000)	(507,000)
Estimated Fair Value	14,471,000	15,347,000
Corporate debt securities
Available-for-sale securities:
Amortized Cost	3,205,000	7,140,000
Gross Unrealized Gain	0	0
Gross Unrealized (Losses)	(152,000)	(208,000)
Estimated Fair Value	3,053,000	$ 6,932,000
Term deposits
Held-to-maturity securities:
Amortized Cost	16,553,000
Gross Unrealized Gain	0
Gross Unrealized (Losses)	0
Estimated Fair Value	$ 16,553,000