Marketable Securities - Available-for-sale Securities by Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Summary of maturities, at par, of marketable securities
2019	$ 38,127	$ 43,667
2020	902	20,123
2021	0	400
Total	39,029	64,190
Certificates of deposit
Summary of maturities, at par, of marketable securities
2019	2,049	2,311
2020	0	1,799
2021	0	0
Total	2,049	4,110
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2019	20,393	17,574
2020	502	9,174
2021	0	0
Total	20,895	26,748
Corporate notes
Summary of maturities, at par, of marketable securities
2019	13,685	18,671
2020	400	7,150
2021	0	400
Total	14,085	26,221
Municipal notes
Summary of maturities, at par, of marketable securities
2019	2,000	5,111
2020	0	2,000
2021	0	0
Total	$ 2,000	$ 7,111