Marketable Investments (Details 3) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Continuous unrealized loss position, Fair Value
Fair Value, Total	$ 85,973	$ 319,401
Continuous unrealized loss position, Gross Unrealized Loss
Gross Unrealized Loss, Total	(52)	(612)
Government-sponsored enterprises
Continuous unrealized loss position, Fair Value
Continuous unrealized loss position less than one year, Fair value	53,646	170,018
Fair Value, Total	53,646	170,018
Continuous unrealized loss position, Gross Unrealized Loss
Continuous unrealized loss position less than one year, Gross Unrealized Loss	(32)	(170)
Gross Unrealized Loss, Total	(32)	(170)
Corporate notes and bonds
Continuous unrealized loss position, Fair Value
Continuous unrealized loss position less than one year, Fair value	32,327	149,383
Fair Value, Total	32,327	149,383
Continuous unrealized loss position, Gross Unrealized Loss
Continuous unrealized loss position less than one year, Gross Unrealized Loss	(20)	(442)
Gross Unrealized Loss, Total	$ (20)	$ (442)