Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified
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Mar. 31, 2015
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Dec. 31, 2014
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Assets: |
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Assets, fair value disclosure, recurring |
$ 469,314us-gaap_AssetsFairValueDisclosureRecurring |
$ 492,003us-gaap_AssetsFairValueDisclosureRecurring |
Liabilities: |
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Liabilities, fair value disclosure, recurring |
7,011us-gaap_LiabilitiesFairValueDisclosureRecurring |
6,796us-gaap_LiabilitiesFairValueDisclosureRecurring |
Contingent consideration obligations [Member] |
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Liabilities: |
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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] |
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Assets: |
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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] |
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Assets: |
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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] |
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Assets: |
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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] |
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Assets: |
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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] |
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Assets: |
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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] |
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Assets: |
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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] |
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Assets: |
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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] |
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Assets: |
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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] |
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Liabilities: |
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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] |
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Liabilities: |
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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 |