Marketable Securities - Available-for-sale Securities by Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Summary of maturities, at par, of marketable securities
2019	$ 32,448	$ 43,667
2020	27,672	20,123
2021	400	400
Total	60,520	64,190
Certificates of deposit
Summary of maturities, at par, of marketable securities
2019	2,212	2,311
2020	1,799	1,799
2021	0	0
Total	4,011	4,110
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2019	13,414	17,574
2020	10,173	9,174
2021	0	0
Total	23,587	26,748
Corporate notes
Summary of maturities, at par, of marketable securities
2019	11,871	18,671
2020	13,700	7,150
2021	400	400
Total	25,971	26,221
Municipal notes
Summary of maturities, at par, of marketable securities
2019	4,951	5,111
2020	2,000	2,000
2021	0	0
Total	$ 6,951	$ 7,111