Marketable Securities - Available-for-sale Securities by Maturities (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Summary of maturities, at par, of marketable securities
2016	$ 5,021	$ 8,159
2017	9,270	8,855
2018	14,267	13,006
2019	3,403	2,238
Total	31,961	32,258
Certificates of deposit
Summary of maturities, at par, of marketable securities
2016	1,079	2,492
2017	671	631
2018	4,510	4,510
2019	169	169
Total	6,429	7,802
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2016	100	100
2017	1,234	759
2018	579	579
2019	143	188
Total	2,056	1,626
Corporate notes
Summary of maturities, at par, of marketable securities
2016	3,067	4,572
2017	6,425	6,525
2018	7,573	6,462
2019	2,861	1,881
Total	19,926	19,440
Municipal notes
Summary of maturities, at par, of marketable securities
2016	775	995
2017	940	940
2018	1,605	1,455
2019	230	0
Total	$ 3,550	$ 3,390