Marketable Securities - Available-for-sale Securities by Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Summary of maturities, at par, of marketable securities
2020	$ 32,888	$ 38,127
2021	1,202	902
Total	34,090	39,029
Certificates of deposit
Summary of maturities, at par, of marketable securities
2020	2,049	2,049
2021	0	0
Total	2,049	2,049
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2020	17,439	20,393
2021	802	502
Total	18,241	20,895
Corporate notes
Summary of maturities, at par, of marketable securities
2020	11,400	13,685
2021	400	400
Total	11,800	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