Investments (Details 4) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Amortized Cost
Due in less than one year, Amortized Cost	$ 346,698
Due in one to two years, Amortized Cost	246,705
Due in three to five years, Amortized Cost	137,062
Total marketable investments	730,465	857,394
Fair Value
Due in less than one year, Fair Value	346,868
Due in one to two years, Fair Value	247,037
Due in three to five years, Fair Value	137,172
Fair Value	$ 731,077	$ 857,711