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Financial Instruments - Location and Pretax Gain or Loss Amounts for Derivatives (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]      
Ineffectiveness on the hedged item $ 3us-gaap_DerivativeNetHedgeIneffectivenessGainLoss $ 2us-gaap_DerivativeNetHedgeIneffectivenessGainLoss  
Derivatives not designated in a hedging relationship [Member] | Foreign exchange contract [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Amount of (gain) loss recognized in Other (income) expense, net on derivatives (516)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 [1] 183us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 [1] 382us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 [1]
Amount of loss recognized in Sales 15mrk_DerivativeInstrumentsGainLossRecognizedInSales
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 8mrk_DerivativeInstrumentsGainLossRecognizedInSales
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 30mrk_DerivativeInstrumentsGainLossRecognizedInSales
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Derivatives designated in fair value hedging relationships [Member] | Interest rate swap contract [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Amount of (gain) loss recognized in Other (income) expense, net on derivatives (17)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember		 [2] 12us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember		 [2] 0us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember		 [2]
Amount of loss (gain) recognized in Other (income) expense, net on hedged item 14us-gaap_IncreaseDecreaseInFairValueOfHedgedItemInInterestRateFairValueHedge1
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember		 [2] (14)us-gaap_IncreaseDecreaseInFairValueOfHedgedItemInInterestRateFairValueHedge1
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember		 [2] 0us-gaap_IncreaseDecreaseInFairValueOfHedgedItemInInterestRateFairValueHedge1
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember		 [2]
Derivatives designated in foreign currency cash flow hedging relationships [Member] | Foreign exchange contract [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Amount of (gain) loss reclassified from AOCI to Sales (143)us-gaap_Revenues
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		 45us-gaap_Revenues
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		 50us-gaap_Revenues
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
Amount of (gain) loss recognized in OCI on derivatives (775)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		 (306)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		 204us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
Derivatives designated in foreign currency net investment hedging relationships [Member] | Foreign exchange contract [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Amount of (gain) loss recognized in OCI on derivatives (192)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_NetInvestmentHedgingMember		 (363)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_NetInvestmentHedgingMember		 (208)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_NetInvestmentHedgingMember
Amount of gain recognized in Other (income) expense, net on derivatives $ (6)us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_NetInvestmentHedgingMember		 [3] $ (10)us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_NetInvestmentHedgingMember		 [3] $ (20)us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_NetInvestmentHedgingMember		 [3]
[1] These derivative contracts mitigate changes in the value of remeasured foreign currency denominated monetary assets and liabilities attributable to changes in foreign currency exchange rates.
[2] There was $3 million and $2 million of ineffectiveness on the hedge during 2014 and 2013, respectively.
[3] There was no ineffectiveness on the hedge. Represents the amount excluded from hedge effectiveness testing.