Marketable Securities - Available-for-sale Securities by Maturities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Summary of maturities, at par, of marketable securities
2017	$ 3,156	$ 6,921
2018	13,511	13,571
2019	5,742	5,773
2020	0	0
Total	22,409	26,265
Certificates of deposit
Summary of maturities, at par, of marketable securities
2017	195	531
2018	4,306	4,306
2019	324	324
2020	0	0
Total	4,825	5,161
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2017	692	1,234
2018	444	444
2019	141	142
2020	0	0
Total	1,277	1,820
Corporate notes
Summary of maturities, at par, of marketable securities
2017	1,724	4,316
2018	7,073	7,133
2019	4,152	4,232
2020	0	0
Total	12,949	15,681
Municipal notes
Summary of maturities, at par, of marketable securities
2017	545	840
2018	1,688	1,688
2019	1,125	1,075
2020	0	0
Total	$ 3,358	$ 3,603