Marketable Securities - Available-for-sale Securities by Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Summary of maturities, at par, of marketable securities
2019	$ 43,667	$ 20,227
2020	20,123	30,297
2021	400	20,423
2022	0	0
Total	64,190	70,947
Certificates of deposit
Summary of maturities, at par, of marketable securities
2019	2,311	4,306
2020	1,799	2,311
2021	0	1,799
2022	0	0
Total	4,110	8,416
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2019	17,574	6,399
2020	9,174	14,599
2021	0	9,171
2022	0	0
Total	26,748	30,169
Corporate notes
Summary of maturities, at par, of marketable securities
2019	18,671	7,954
2020	7,150	6,430
2021	400	6,450
2022	0	0
Total	26,221	20,834
Municipal notes
Summary of maturities, at par, of marketable securities
2019	5,111	1,568
2020	2,000	6,957
2021	0	3,003
2022	0	0
Total	$ 7,111	$ 11,528