Marketable Investments (Details 4) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Amortized Cost
Due in less than one year, Amortized Cost	$ 409,645
Due in one to two years, Amortized Cost	325,818
Due in three to five years, Amortized Cost	121,931
Total marketable investments	857,394	630,428
Fair Value
Due in less than one year, Fair Value	409,751
Due in one to two years, Fair Value	325,960
Due in three to five years, Fair Value	122,000
Fair Value	$ 857,711	$ 630,698