Marketable Securities - Available-for-sale Securities by Maturities (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Summary of maturities, at par, of marketable securities
2015	$ 900	$ 17,082
2016	8,189	10,040
2017	9,055	9,328
2018	12,814	4,778
2019	1,988
Total	32,946	41,228
Certificates of deposit
Summary of maturities, at par, of marketable securities
2015	395	4,213
2016	2,492	1,501
2017	631	831
2018	4,510	2,149
2019	0
Total	8,028	8,694
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2015	0	1,176
2016	100	600
2017	959	1,209
2018	579	879
2019	193
Total	1,831	3,864
Corporate notes
Summary of maturities, at par, of marketable securities
2015	305	9,588
2016	4,572	6,704
2017	6,525	6,498
2018	6,270	1,625
2019	1,795
Total	19,467	24,415
Municipal notes
Summary of maturities, at par, of marketable securities
2015	200	2,105
2016	1,025	1,235
2017	940	790
2018	1,455	125
2019	0
Total	$ 3,620	$ 4,255