Marketable Investments (Details 3) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Continuous unrealized loss position, Fair Value
Fair Value, Total	$ 163,687	$ 319,401
Continuous unrealized loss position, Gross Unrealized Loss
Gross Unrealized Loss, Total	(175)	(612)
Government-sponsored enterprises
Continuous unrealized loss position, Fair Value
Less than one year, Fair value	72,727	170,018
Fair Value, Total	72,727	170,018
Continuous unrealized loss position, Gross Unrealized Loss
Less than one year, Gross Unrealized Loss	(35)	(170)
Gross Unrealized Loss, Total	(35)	(170)
Corporate notes and bonds
Continuous unrealized loss position, Fair Value
Less than one year, Fair value	90,960	149,383
Fair Value, Total	90,960	149,383
Continuous unrealized loss position, Gross Unrealized Loss
Less than one year, Gross Unrealized Loss	(140)	(442)
Gross Unrealized Loss, Total	$ (140)	$ (442)