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Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Assets:    
Assets, fair value disclosure, recurring $ 469,314us-gaap_AssetsFairValueDisclosureRecurring $ 492,003us-gaap_AssetsFairValueDisclosureRecurring
Liabilities:    
Liabilities, fair value disclosure, recurring 7,011us-gaap_LiabilitiesFairValueDisclosureRecurring 6,796us-gaap_LiabilitiesFairValueDisclosureRecurring
Contingent consideration obligations [Member]    
Liabilities:    
Liabilities, fair value disclosure, recurring 7,011us-gaap_LiabilitiesFairValueDisclosureRecurring
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_ObligationsMember
6,796us-gaap_LiabilitiesFairValueDisclosureRecurring
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_ObligationsMember
Cash and Cash Equivalents [Member]    
Assets:    
Assets, fair value disclosure, recurring 133,174us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CashAndCashEquivalentsMember
347,845us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CashAndCashEquivalentsMember
U.S. Government Agency Securities [Member]    
Assets:    
Assets, fair value disclosure, recurring 323,741us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
131,536us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Certificates of Deposit [Member]    
Assets:    
Assets, fair value disclosure, recurring 12,399us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CertificatesOfDepositMember
12,622us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CertificatesOfDepositMember
Quoted prices in active markets (Level 1) [Member]    
Assets:    
Assets, fair value disclosure, recurring 133,174us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
347,845us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Quoted prices in active markets (Level 1) [Member] | Cash and Cash Equivalents [Member]    
Assets:    
Assets, fair value disclosure, recurring 133,174us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CashAndCashEquivalentsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
347,845us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CashAndCashEquivalentsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Significant other observable inputs (Level 2) [Member]    
Assets:    
Assets, fair value disclosure, recurring 336,140us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
144,158us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Significant other observable inputs (Level 2) [Member] | U.S. Government Agency Securities [Member]    
Assets:    
Assets, fair value disclosure, recurring 323,741us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
131,536us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Significant other observable inputs (Level 2) [Member] | Certificates of Deposit [Member]    
Assets:    
Assets, fair value disclosure, recurring 12,399us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CertificatesOfDepositMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
12,622us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CertificatesOfDepositMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Significant unobservable inputs (Level 3) [Member]    
Liabilities:    
Liabilities, fair value disclosure, recurring 7,011us-gaap_LiabilitiesFairValueDisclosureRecurring
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
6,796us-gaap_LiabilitiesFairValueDisclosureRecurring
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Significant unobservable inputs (Level 3) [Member] | Contingent consideration obligations [Member]    
Liabilities:    
Liabilities, fair value disclosure, recurring $ 7,011us-gaap_LiabilitiesFairValueDisclosureRecurring
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_ObligationsMember
$ 6,796us-gaap_LiabilitiesFairValueDisclosureRecurring
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_ObligationsMember