Marketable Securities - Available-for-sale Securities by Maturities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Summary of maturities, at par, of marketable securities
2017	$ 2,106	$ 6,921
2018	13,511	13,571
2019	5,638	5,773
2020	0	0
Total	21,255	26,265
Certificates of deposit
Summary of maturities, at par, of marketable securities
2017	145	531
2018	4,306	4,306
2019	324	324
2020	0	0
Total	4,775	5,161
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2017	592	1,234
2018	444	444
2019	142	142
2020	0	0
Total	1,178	1,820
Corporate notes
Summary of maturities, at par, of marketable securities
2017	1,124	4,316
2018	7,073	7,133
2019	4,065	4,232
2020	0	0
Total	12,262	15,681
Municipal notes
Summary of maturities, at par, of marketable securities
2017	245	840
2018	1,688	1,688
2019	1,107	1,075
2020	0	0
Total	$ 3,040	$ 3,603