Investments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Marketable investments classified as held-to-maturity:
Amortized Cost	$ 420,500us-gaap_HeldToMaturitySecurities	$ 857,394us-gaap_HeldToMaturitySecurities
Gross Unrealized Gains	378	557
Gross Unrealized Losses	(147)	(240)
Fair Value	420,731	857,711
Government-sponsored enterprises
Marketable investments classified as held-to-maturity:
Amortized Cost	127,212	445,939
Gross Unrealized Gains	118	257
Gross Unrealized Losses	(39)	(77)
Fair Value	127,291	446,119
Corporate notes and bonds
Marketable investments classified as held-to-maturity:
Amortized Cost	293,288	411,455
Gross Unrealized Gains	260	300
Gross Unrealized Losses	(108)	(163)
Fair Value	$ 293,440	$ 411,592

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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