Summary of Significant Accounting Policies (Details 4) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2013 Item	Dec. 31, 2012 Item
U.S. Government-sponsored enterprise obligations
Unrealized loss position for less than one year
Aggregate Fair Value	$ 39,822
Unrealized Losses	(12)
Corporate debt securities
Unrealized loss position for less than one year
Number of marketable securities in an unrealized loss position for less than one year	1	28
Specified period for which marketable securities remain in a continuous unrealized loss position	P14M
Number of marketable securities		0
Aggregate Fair Value	84,402	83,529
Unrealized Losses	(68)	(41)
Foreign government bonds
Unrealized loss position for less than one year
Aggregate Fair Value	6,657	8,445
Unrealized Losses	$ (1)	$ (3)

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

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Disclosure regarding factors used to determine that the impairment of securities not categorized as either held-to-maturity securities or trading securities (hence equal to available for sale securities) where cost exceeds fair value is not an other than temporary impairment (OTTI). This item contains disclosure of the number of investment positions in the available-for-sale investments determined to be temporarily impaired.

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- Definition

Disclosure regarding factors used to determine that the impairment of securities not categorized as either held-to-maturity securities or trading securities (hence equal to available for sale securities) where cost exceeds fair value is not an other than temporary impairment (OTTI). This item contains disclosure of the severity and duration of the temporary impairment of the available-for-sale investments.

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