Marketable Securities - Available-for-sale Securities by Maturities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Summary of maturities, at par, of marketable securities
2015	$ 17,082	$ 17,166
2016	10,040	19,791
2017	9,328	10,040
2018	4,778	6,842
Total	41,228	53,839
Certificates of deposit
Summary of maturities, at par, of marketable securities
2015	4,213	1,627
2016	1,501	4,213
2017	831	1,501
2018	2,149	681
Total	8,694	8,022
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2015	1,176	5,485
2016	600	3,336
2017	1,209	600
2018	879	692
Total	3,864	10,113
Corporate notes
Summary of maturities, at par, of marketable securities
2015	9,588	6,729
2016	6,704	10,037
2017	6,498	6,704
2018	1,625	5,174
Total	24,415	28,644
Municipal notes
Summary of maturities, at par, of marketable securities
2015	2,105	3,325
2016	1,235	2,205
2017	790	1,235
2018	125	295
Total	$ 4,255	$ 7,060