The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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+ Details

Summary of Significant Accounting Policies (Details 2) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2013 Recurring Fair value	Dec. 31, 2012 Recurring Fair value	Mar. 31, 2013 Recurring Fair value Cash and cash equivalents	Dec. 31, 2012 Recurring Fair value Cash and cash equivalents	Mar. 31, 2013 Recurring Fair value U.S. Government-sponsored enterprise obligations	Dec. 31, 2012 Recurring Fair value U.S. Government-sponsored enterprise obligations	Mar. 31, 2013 Recurring Fair value Corporate debt securities	Dec. 31, 2012 Recurring Fair value Corporate debt securities	Mar. 31, 2013 Recurring Fair value Commercial paper obligations	Dec. 31, 2012 Recurring Fair value Commercial paper obligations	Mar. 31, 2013 Recurring Fair value Foreign government bonds	Dec. 31, 2012 Recurring Fair value Foreign government bonds	Mar. 31, 2013 Recurring Quoted Prices in Active Markets (Level 1)	Dec. 31, 2012 Recurring Quoted Prices in Active Markets (Level 1)	Mar. 31, 2013 Recurring Quoted Prices in Active Markets (Level 1) Cash and cash equivalents	Dec. 31, 2012 Recurring Quoted Prices in Active Markets (Level 1) Cash and cash equivalents	Mar. 31, 2013 Recurring Significant Other Observable Inputs (Level 2)	Dec. 31, 2012 Recurring Significant Other Observable Inputs (Level 2)	Mar. 31, 2013 Recurring Significant Other Observable Inputs (Level 2) U.S. Government-sponsored enterprise obligations	Dec. 31, 2012 Recurring Significant Other Observable Inputs (Level 2) U.S. Government-sponsored enterprise obligations	Mar. 31, 2013 Recurring Significant Other Observable Inputs (Level 2) Corporate debt securities	Dec. 31, 2012 Recurring Significant Other Observable Inputs (Level 2) Corporate debt securities	Mar. 31, 2013 Recurring Significant Other Observable Inputs (Level 2) Commercial paper obligations	Dec. 31, 2012 Recurring Significant Other Observable Inputs (Level 2) Commercial paper obligations	Mar. 31, 2013 Recurring Significant Other Observable Inputs (Level 2) Foreign government bonds	Dec. 31, 2012 Recurring Significant Other Observable Inputs (Level 2) Foreign government bonds
Fair Value Measurements
Assets fair value		$ 319,764,000	$ 335,553,000	$ 38,848,000	$ 47,940,000	$ 67,788,000	$ 51,225,000	$ 145,526,000	$ 158,913,000	$ 57,633,000	$ 65,138,000	$ 9,969,000	$ 12,337,000	$ 38,848,000	$ 47,940,000	$ 38,848,000	$ 47,940,000	$ 280,916,000	$ 287,613,000	$ 67,788,000	$ 51,225,000	$ 145,526,000	$ 158,913,000	$ 57,633,000	$ 65,138,000	$ 9,969,000	$ 12,337,000
Transfer from Level 1 to Level 2, assets	0
Transfer from Level 2 to Level 1, assets	0
Other-than-temporary impairment	$ 0