Investments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Marketable investments classified as held-to-maturity:
Amortized Cost	$ 476,058	$ 857,394
Gross Unrealized Gains	693	557
Gross Unrealized Losses	(84)	(240)
Fair Value	476,667	857,711
US Government-sponsored Enterprises Debt Securities [Member]
Marketable investments classified as held-to-maturity:
Amortized Cost	151,256	445,939
Gross Unrealized Gains	188	257
Gross Unrealized Losses	(18)	(77)
Fair Value	151,426	446,119
Corporate Debt Securities [Member]
Marketable investments classified as held-to-maturity:
Amortized Cost	324,802	411,455
Gross Unrealized Gains	505	300
Gross Unrealized Losses	(66)	(163)
Fair Value	$ 325,241	$ 411,592