Marketable Securities - Available-for-sale Securities by Maturities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Summary of maturities, at par, of marketable securities
2019	$ 24,624	$ 43,667
2020	31,057	20,123
2021	900	400
Total	56,581	64,190
Certificates of deposit
Summary of maturities, at par, of marketable securities
2019	2,459	2,311
2020	2,049	1,799
2021	0	0
Total	4,508	4,110
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2019	13,566	17,574
2020	12,323	9,174
2021	500	0
Total	26,389	26,748
Corporate notes
Summary of maturities, at par, of marketable securities
2019	3,923	18,671
2020	14,685	7,150
2021	400	400
Total	19,008	26,221
Municipal notes
Summary of maturities, at par, of marketable securities
2019	4,676	5,111
2020	2,000	2,000
2021	0	0
Total	$ 6,676	$ 7,111