Marketable Securities - Available-for-sale Securities by Maturities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Summary of maturities, at par, of marketable securities
2014	$ 17,166	$ 20,774
2015	19,791	19,944
2016	10,040	18,329
2017	6,842	4,037
Total	53,839	63,084
Certificates of deposit
Summary of maturities, at par, of marketable securities
2014	1,627	1,268
2015	4,213	1,627
2016	1,501	4,316
2017	681	301
Total	8,022	7,512
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2014	5,485	6,285
2015	3,336	7,248
2016	600	2,633
2017	692	11
Total	10,113	16,177
Corporate notes
Summary of maturities, at par, of marketable securities
2014	6,729	10,916
2015	10,037	6,729
2016	6,704	9,420
2017	5,174	3,325
Total	28,644	30,390
Municipal notes
Summary of maturities, at par, of marketable securities
2014	3,325	2,305
2015	2,205	4,340
2016	1,235	1,960
2017	295	400
Total	$ 7,060	$ 9,005