Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Marketable securities by security type
Cost	$ 86,751	$ 89,868
Gross Unrealized Gains	8	14
Gross Unrealized Losses	(18)	(11)
Estimated Fair Value	86,741	89,871
Commercial paper and corporate bonds [Member]
Marketable securities by security type
Cost	45,299	63,682
Gross Unrealized Gains	0	4
Gross Unrealized Losses	(18)	(11)
Estimated Fair Value	45,281	63,675
U.S. Treasury and U.S. government agency debt securities [member]
Marketable securities by security type
Cost	41,452	26,186
Gross Unrealized Gains	8	10
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 41,460	$ 26,196

Amount of debt securities categorized neither as held-to-maturity nor trading.

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Reference 9: http://www.xbrl.org/2003/role/presentationRef

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 10: http://www.xbrl.org/2003/role/presentationRef

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+ Details

- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References+ Details