Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

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- Definition

Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

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-Section 50

-Paragraph 2

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

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Summary of Significant Accounting Policies (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Fair Value Measurements
Transfer from Level 1 to Level 2, assets	$ 0
Transfer from Level 2 to Level 1, assets	0
Other-than-temporary impairment	0us-gaap_ImpairmentOfInvestments	0us-gaap_ImpairmentOfInvestments
Recurring \| Fair value
Fair Value Measurements
Assets fair value	175,895		186,443
Recurring \| Fair value \| Money market funds
Fair Value Measurements
Assets fair value	74,799		55,283
Recurring \| Fair value \| Corporate debt securities
Fair Value Measurements
Assets fair value	46,889		71,648
Recurring \| Fair value \| Commercial paper obligations
Fair Value Measurements
Assets fair value	6,649		15,250
Recurring \| Fair value \| Foreign government bonds
Fair Value Measurements
Assets fair value	21,487		18,520
Recurring \| Fair value \| Asset-backed securities
Fair Value Measurements
Assets fair value	26,071		25,742
Recurring \| Quoted Prices in Active Markets (Level 1)
Fair Value Measurements
Assets fair value	74,799		55,283
Recurring \| Quoted Prices in Active Markets (Level 1) \| Money market funds
Fair Value Measurements
Assets fair value	74,799		55,283
Recurring \| Significant Other Observable Inputs (Level 2)
Fair Value Measurements
Assets fair value	101,096		131,160
Recurring \| Significant Other Observable Inputs (Level 2) \| Corporate debt securities
Fair Value Measurements
Assets fair value	46,889		71,648
Recurring \| Significant Other Observable Inputs (Level 2) \| Commercial paper obligations
Fair Value Measurements
Assets fair value	6,649		15,250
Recurring \| Significant Other Observable Inputs (Level 2) \| Foreign government bonds
Fair Value Measurements
Assets fair value	21,487		18,520
Recurring \| Significant Other Observable Inputs (Level 2) \| Asset-backed securities
Fair Value Measurements
Assets fair value	$ 26,071		$ 25,742