Marketable Securities - Available-for-sale Securities by Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Summary of maturities, at par, of marketable securities
2017	$ 6,921	$ 8,159
2018	13,571	8,855
2019	5,773	13,006
2020	0	2,238
Total	26,265	32,258
Certificates of deposit
Summary of maturities, at par, of marketable securities
2017	531	2,492
2018	4,306	631
2019	324	4,510
2020	0	169
Total	5,161	7,802
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2017	1,234	100
2018	444	759
2019	142	579
2020	0	188
Total	1,820	1,626
Corporate notes
Summary of maturities, at par, of marketable securities
2017	4,316	4,572
2018	7,133	6,525
2019	4,232	6,462
2020	0	1,881
Total	15,681	19,440
Municipal notes
Summary of maturities, at par, of marketable securities
2017	840	995
2018	1,688	940
2019	1,075	1,455
2020	0	0
Total	$ 3,603	$ 3,390