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Fair Value - Schedule of fair value of financial assets and liabilities (Details) (USD $)
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Mar. 31, 2015
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Dec. 31, 2014
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| Assets: |
|
|
| Total assets measured at fair value |
$ 387,384,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
|
$ 386,344,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
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| Money market funds |
|
|
| Assets: |
|
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| Total assets measured at fair value |
93,160,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
|
89,113,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
|
| Commercial paper |
|
|
| Assets: |
|
|
| Total assets measured at fair value |
52,834,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommercialPaperMember
|
54,792,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommercialPaperMember
|
| Corporate debt securities |
|
|
| Assets: |
|
|
| Total assets measured at fair value |
233,643,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateDebtSecuritiesMember
|
234,697,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateDebtSecuritiesMember
|
| U.S. government and government agency debt securities |
|
|
| Assets: |
|
|
| Total assets measured at fair value |
7,747,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
|
7,742,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
|
| Quoted Prices in Active Markets for Identical Instruments (Level 1) |
|
|
| Assets: |
|
|
| Total assets measured at fair value |
93,160,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
|
89,113,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
|
| Quoted Prices in Active Markets for Identical Instruments (Level 1) | Money market funds |
|
|
| Assets: |
|
|
| Total assets measured at fair value |
93,160,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
|
89,113,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
|
| Quoted Prices in Active Markets for Identical Instruments (Level 1) | Commercial paper |
|
|
| Assets: |
|
|
| Total assets measured at fair value |
0p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommercialPaperMember
|
0p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommercialPaperMember
|
| Quoted Prices in Active Markets for Identical Instruments (Level 1) | Corporate debt securities |
|
|
| Assets: |
|
|
| Total assets measured at fair value |
0p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateDebtSecuritiesMember
|
0p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateDebtSecuritiesMember
|
| Quoted Prices in Active Markets for Identical Instruments (Level 1) | U.S. government and government agency debt securities |
|
|
| Assets: |
|
|
| Total assets measured at fair value |
0p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
|
0p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
|
| Significant Other Observable Inputs (Level 2) |
|
|
| Assets: |
|
|
| Total assets measured at fair value |
294,224,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
|
297,231,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
|
| Significant Other Observable Inputs (Level 2) | Money market funds |
|
|
| Assets: |
|
|
| Total assets measured at fair value |
0p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
|
0p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
|
| Significant Other Observable Inputs (Level 2) | Commercial paper |
|
|
| Assets: |
|
|
| Total assets measured at fair value |
52,834,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommercialPaperMember
|
54,792,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommercialPaperMember
|
| Significant Other Observable Inputs (Level 2) | Corporate debt securities |
|
|
| Assets: |
|
|
| Total assets measured at fair value |
233,643,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateDebtSecuritiesMember
|
234,697,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateDebtSecuritiesMember
|
| Significant Other Observable Inputs (Level 2) | U.S. government and government agency debt securities |
|
|
| Assets: |
|
|
| Total assets measured at fair value |
$ 7,747,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
|
$ 7,742,000p_CashEquivalentsAndShortTermInvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
|