Marketable Securities - Available-for-sale Securities by Maturities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Summary of maturities, at par, of marketable securities
2020	$ 24,420	$ 38,127
2021	2,752	902
Total	27,172	39,029
Certificates of deposit
Summary of maturities, at par, of marketable securities
2020	1,799	2,049
2021	0	0
Total	1,799	2,049
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2020	13,821	20,393
2021	802	502
Total	14,623	20,895
Corporate notes
Summary of maturities, at par, of marketable securities
2020	6,800	13,685
2021	1,950	400
Total	8,750	14,085
Municipal notes
Summary of maturities, at par, of marketable securities
2020	2,000	2,000
2021	0	0
Total	$ 2,000	$ 2,000