Investments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Marketable investments classified as held-to-maturity:
Amortized Cost	$ 730,465	$ 857,394
Gross Unrealized Gains	806	557
Gross Unrealized Losses	(194)	(240)
Fair Value	731,077	857,711
Government-sponsored enterprises
Marketable investments classified as held-to-maturity:
Amortized Cost	316,197	445,939
Gross Unrealized Gains	284	257
Gross Unrealized Losses	(69)	(77)
Fair Value	316,412	446,119
Corporate notes and bonds
Marketable investments classified as held-to-maturity:
Amortized Cost	414,268	411,455
Gross Unrealized Gains	522	300
Gross Unrealized Losses	(125)	(163)
Fair Value	$ 414,665	$ 411,592