Marketable Securities - Available-for-sale Securities by Maturities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Summary of maturities, at par, of marketable securities
2018	$ 10,710	$ 20,227
2019	38,717	30,297
2020	20,123	20,423
2021	400	0
Total	69,950	70,947
Certificates of deposit
Summary of maturities, at par, of marketable securities
2018	2,211	4,306
2019	2,311	2,311
2020	1,799	1,799
2021	0	0
Total	6,321	8,416
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2018	3,351	6,399
2019	17,574	14,599
2020	9,174	9,171
2021	0	0
Total	30,099	30,169
Corporate notes
Summary of maturities, at par, of marketable securities
2018	4,325	7,954
2019	13,721	6,430
2020	7,150	6,450
2021	400	0
Total	25,596	20,834
Municipal notes
Summary of maturities, at par, of marketable securities
2018	823	1,568
2019	5,111	6,957
2020	2,000	3,003
2021	0	0
Total	$ 7,934	$ 11,528