Marketable Securities (Maturities, at par) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Summary of maturities, at par, of marketable securities
2013	$ 9,144	$ 20,774
2014	20,008	19,944
2015	19,883	18,329
2016	9,836	4,037
2017	4,267
Total	63,138	63,084
Certificates of deposit
Summary of maturities, at par, of marketable securities
2013	1,255	1,268
2014	1,627	1,627
2015	4,213	4,316
2016	1,301	301
2017	97
Total	8,493	7,512
US Treasury and agency notes
Summary of maturities, at par, of marketable securities
2013	3,566	6,285
2014	7,287	7,248
2015	3,448	2,633
2016	1,750	11
2017	391
Total	16,442	16,177
Corporate notes
Summary of maturities, at par, of marketable securities
2013	2,988	10,916
2014	6,729	6,729
2015	10,017	9,420
2016	5,650	3,325
2017	3,574
Total	28,958	30,390
Municipal notes
Summary of maturities, at par, of marketable securities
2013	1,335	2,305
2014	4,365	4,340
2015	2,205	1,960
2016	1,135	400
2017	205
Total	$ 9,245	$ 9,005