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Fair Value Measurements - Summary of Placement in the Fair Value Hierarchy of Financial Assets that are Measured at Fair Value on a Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Assets |  |
Assets, Fair Value Disclosure | $ 387,188 us-gaap_AssetsFairValueDisclosure | $ 280,497 us-gaap_AssetsFairValueDisclosure
U.S. government debt securities |  |
Assets |  |
Assets, Fair Value Disclosure | 91,238 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_USGovernmentDebtSecuritiesMember | 115,335 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_USGovernmentDebtSecuritiesMember
Equity Securities |  |
Assets |  |
Assets, Fair Value Disclosure | 294,922 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_EquitySecuritiesMember | 164,889 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_EquitySecuritiesMember
Other Financial Assets |  |
Assets |  |
Assets, Fair Value Disclosure | 1,028 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = xon_OtherFinancialAssetsMember | 273 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = xon_OtherFinancialAssetsMember
Quoted Prices in Active Markets (Level 1) |  |
Assets |  |
Assets, Fair Value Disclosure | 277,903 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 143,927 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Quoted Prices in Active Markets (Level 1) | U.S. government debt securities |  |
Assets |  |
Assets, Fair Value Disclosure | 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_USGovernmentDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_USGovernmentDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Quoted Prices in Active Markets (Level 1) | Equity Securities |  |
Assets |  |
Assets, Fair Value Disclosure | 277,903 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 143,927 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Quoted Prices in Active Markets (Level 1) | Other Financial Assets |  |
Assets |  |
Assets, Fair Value Disclosure | 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = xon_OtherFinancialAssetsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = xon_OtherFinancialAssetsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Significant Other Observable Inputs (Level 2) |  |
Assets |  |
Assets, Fair Value Disclosure | 109,285 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 136,570 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Significant Other Observable Inputs (Level 2) | U.S. government debt securities |  |
Assets |  |
Assets, Fair Value Disclosure | 91,238 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_USGovernmentDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 115,335 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_USGovernmentDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Significant Other Observable Inputs (Level 2) | Equity Securities |  |
Assets |  |
Assets, Fair Value Disclosure | 17,019 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 20,962 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Significant Other Observable Inputs (Level 2) | Other Financial Assets |  |
Assets |  |
Assets, Fair Value Disclosure | 1,028 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = xon_OtherFinancialAssetsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 273 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = xon_OtherFinancialAssetsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Significant Unobservable Inputs (Level 3) |  |
Assets |  |
Assets, Fair Value Disclosure | 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Significant Unobservable Inputs (Level 3) | U.S. government debt securities |  |
Assets |  |
Assets, Fair Value Disclosure | 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_USGovernmentDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_USGovernmentDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Significant Unobservable Inputs (Level 3) | Equity Securities |  |
Assets |  |
Assets, Fair Value Disclosure | 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Significant Unobservable Inputs (Level 3) | Other Financial Assets |  |
Assets |  |
Assets, Fair Value Disclosure | $ 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = xon_OtherFinancialAssetsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | $ 0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = xon_OtherFinancialAssetsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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