Marketable Investments (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Continuous unrealized loss position, Fair Value
Fair Value, Total	$ 245,320	$ 163,687
Continuous unrealized loss position, Gross Unrealized Loss
Gross Unrealized Loss, Total	(240)	(175)
Maturities of securities in excess of which the entity does not invest	3 years
Government-sponsored enterprises
Continuous unrealized loss position, Fair Value
Less than one year, Fair value	76,651	72,727
Fair Value, Total	76,651	72,727
Continuous unrealized loss position, Gross Unrealized Loss
Less than one year, Gross Unrealized Loss	(77)	(35)
Gross Unrealized Loss, Total	(77)	(35)
Corporate notes and bonds
Continuous unrealized loss position, Fair Value
Less than one year, Fair value	168,669	90,960
Fair Value, Total	168,669	90,960
Continuous unrealized loss position, Gross Unrealized Loss
Less than one year, Gross Unrealized Loss	(163)	(140)
Gross Unrealized Loss, Total	$ (163)	$ (140)