Cash, Cash Equivalents and Investments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Components of cash, cash equivalents and investments
Cash	$ 36,404	$ 26,650
Money market instruments	1,699	6,553
Cash and cash equivalents	38,103	33,203	119,831	127,138
Short-term investments	144,431	147,823
Limited partnership interest (accounted for using cost method)	5,933	5,933
Long-term investments	174,906	102,055
Total	357,440	283,081
Treasury bills [Member]
Components of cash, cash equivalents and investments
Short-term investments	1,945	2,494
Municipal bonds [Member]
Components of cash, cash equivalents and investments
Short-term investments	54,036	75,457
Long-term investments	69,680	34,794
Corporate bonds [Member]
Components of cash, cash equivalents and investments
Short-term investments	55,570	34,543
Long-term investments	59,902	36,762
Agency bonds [Member]
Components of cash, cash equivalents and investments
Short-term investments	27,545	15,979
Long-term investments	12,335	21,025
Sovereign bonds [Member]
Components of cash, cash equivalents and investments
Short-term investments	4,016	19,350
Long-term investments	22,355	0
Covered bonds [Member]
Components of cash, cash equivalents and investments
Short-term investments	1,319	0
Long-term investments	$ 4,701	$ 3,541