Marketable Securities - Available-for-sale Securities by Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Summary of maturities, at par, of marketable securities
2018	$ 20,227	$ 6,921
2019	30,297	13,571
2020	20,423	5,773
2021	0	0
Total	70,947	26,265
Certificates of deposit
Summary of maturities, at par, of marketable securities
2018	4,306	531
2019	2,311	4,306
2020	1,799	324
2021	0	0
Total	8,416	5,161
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2018	6,399	1,234
2019	14,599	444
2020	9,171	142
2021	0	0
Total	30,169	1,820
Corporate notes
Summary of maturities, at par, of marketable securities
2018	7,954	4,316
2019	6,430	7,133
2020	6,450	4,232
2021	0	0
Total	20,834	15,681
Municipal notes
Summary of maturities, at par, of marketable securities
2018	1,568	840
2019	6,957	1,688
2020	3,003	1,075
2021	0	0
Total	$ 11,528	$ 3,603