Marketable Securities - Available-for-sale Securities by Maturities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Summary of maturities, at par, of marketable securities
2018	$ 16,013	$ 20,227
2019	34,013	30,297
2020	20,123	20,423
2021	400	0
Total	70,549	70,947
Certificates of deposit
Summary of maturities, at par, of marketable securities
2018	3,497	4,306
2019	2,311	2,311
2020	1,799	1,799
2021	0	0
Total	7,607	8,416
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2018	4,176	6,399
2019	16,924	14,599
2020	9,174	9,171
2021	0	0
Total	30,274	30,169
Corporate notes
Summary of maturities, at par, of marketable securities
2018	7,097	7,954
2019	9,621	6,430
2020	7,150	6,450
2021	400	0
Total	24,268	20,834
Municipal notes
Summary of maturities, at par, of marketable securities
2018	1,243	1,568
2019	5,157	6,957
2020	2,000	3,003
2021	0	0
Total	$ 8,400	$ 11,528