Marketable Securities - Available-for-sale Securities by Maturities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Summary of maturities, at par, of marketable securities
2014	$ 12,566	$ 17,166
2015	19,691	19,791
2016	10,040	10,040
2017	8,416	6,842
Available for Sale Securities Maturities, Year Five, at Par	656
Total	51,369	53,839
Certificates of deposit
Summary of maturities, at par, of marketable securities
2014	1,437	1,627
2015	4,213	4,213
2016	1,501	1,501
2017	731	681
Available for Sale Securities Maturities, Year Five, at Par	409
Total	8,291	8,022
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2014	4,575	5,485
2015	3,336	3,336
2016	600	600
2017	1,092	692
Available for Sale Securities Maturities, Year Five, at Par	0
Total	9,603	10,113
Corporate notes
Summary of maturities, at par, of marketable securities
2014	3,769	6,729
2015	9,937	10,037
2016	6,704	6,704
2017	6,098	5,174
Available for Sale Securities Maturities, Year Five, at Par	247
Total	26,755	28,644
Municipal notes
Summary of maturities, at par, of marketable securities
2014	2,785	3,325
2015	2,205	2,205
2016	1,235	1,235
2017	495	295
Available for Sale Securities Maturities, Year Five, at Par	0
Total	$ 6,720	$ 7,060