Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Marketable investments classified as held-to-maturity:
Amortized Cost	$ 159,976	$ 420,500
Gross Unrealized Gains	40	378
Gross Unrealized Losses	(186)	(147)
Fair Value	159,830	420,731
Government-sponsored enterprises
Marketable investments classified as held-to-maturity:
Amortized Cost	53,254	127,212
Gross Unrealized Gains	3	118
Gross Unrealized Losses	(119)	(39)
Fair Value	53,138	127,291
Corporate notes and bonds
Marketable investments classified as held-to-maturity:
Amortized Cost	106,722	293,288
Gross Unrealized Gains	37	260
Gross Unrealized Losses	(67)	(108)
Fair Value	$ 106,692	$ 293,440

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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