Marketable Securities - Available-for-sale Securities by Major Security Type (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Marketable Securities
Amortized cost	$ 3,007	$ 2,848
Gross unrealized holding gains	8	31
Gross unrealized holding losses	(5)
Estimated fair value	3,010	2,879
Corporate bonds and notes
Marketable Securities
Amortized cost	1,434	1,312
Gross unrealized holding gains	8	26
Gross unrealized holding losses	(2)
Estimated fair value	1,440	1,338
Municipal bonds
Marketable Securities
Amortized cost	1,573	1,536
Gross unrealized holding gains		5
Gross unrealized holding losses	(3)
Estimated fair value	$ 1,570	$ 1,541