Marketable Investments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Marketable investments classified as held-to-maturity:
Amortized Cost	$ 857,394	$ 630,428
Gross Unrealized Gains	557	445
Gross Unrealized Losses	(240)	(175)
Fair Value	857,711	630,698
Government-sponsored enterprises
Marketable investments classified as held-to-maturity:
Amortized Cost	445,939	350,043
Gross Unrealized Gains	257	261
Gross Unrealized Losses	(77)	(35)
Fair Value	446,119	350,269
Corporate notes and bonds
Marketable investments classified as held-to-maturity:
Amortized Cost	411,455	280,385
Gross Unrealized Gains	300	184
Gross Unrealized Losses	(163)	(140)
Fair Value	$ 411,592	$ 280,429