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Fair Value (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Fair Value [Line Items]    
Transfers from level 1 to level 2-Assets $ 0us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount $ 0us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount
Transfers from level 1 to level 2-Liabilities 0us-gaap_FairValueLiabilitiesLevel1ToLevel2TransfersAmount 0us-gaap_FairValueLiabilitiesLevel1ToLevel2TransfersAmount
Transfer from level 2 to level 1-Assets 0us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount 0us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount
Transfer from level 2 to level 1-Liabilities 0us-gaap_FairValueLiabilitiesLevel2ToLevel1TransfersAmount 0us-gaap_FairValueLiabilitiesLevel2ToLevel1TransfersAmount
Transfers Into Level 3 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3
Transfers out of Level 3 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3
Fair Value, Measurements, Nonrecurring [Member]    
Fair Value [Line Items]    
Significant fair value adjustments for assets and liabilities measured on a nonrecurring basis 0unh_FairValueAdjustmentsForNonfinancialAssetsAndLiabilitiesMeasuredAtFairValueOrFinancialAssetsandLiabilitiesNonRecurringBasis
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsNonrecurringMember
0unh_FairValueAdjustmentsForNonfinancialAssetsAndLiabilitiesMeasuredAtFairValueOrFinancialAssetsandLiabilitiesNonRecurringBasis
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsNonrecurringMember
Fair Value, Measurements, Recurring [Member] | Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value [Line Items]    
Available-for-sale Securities $ 74us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember