Marketable Securities - Available-for-sale Securities by Maturities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Summary of maturities, at par, of marketable securities
Year 1	$ 20,774	$ 10,580
Year 2	19,944	25,276
Year 3	18,329	20,551
Year 4	4,037	10,206
Year 5		0
Total	63,084	66,613
Certificates of deposit [Member]
Summary of maturities, at par, of marketable securities
Year 1	1,268	1,536
Year 2	1,627	1,255
Year 3	4,316	1,627
Year 4	301	2,526
Year 5		0
Total	7,512	6,944
U.S. Treasury and agency notes [Member]
Summary of maturities, at par, of marketable securities
Year 1	6,285	4,734
Year 2	7,248	10,285
Year 3	2,633	5,856
Year 4	11	936
Year 5		0
Total	16,177	21,811
Corporate notes [Member]
Summary of maturities, at par, of marketable securities
Year 1	10,916	3,450
Year 2	6,729	11,231
Year 3	9,420	8,923
Year 4	3,325	5,584
Year 5		0
Total	30,390	29,188
Municipal notes [Member]
Summary of maturities, at par, of marketable securities
Year 1	2,305	860
Year 2	4,340	2,505
Year 3	1,960	4,145
Year 4	400	1,160
Year 5		0
Total	$ 9,005	$ 8,670