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Equity - Textual (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 1998
Common stock, shares authorized 6,500,000,000us-gaap_CommonStockSharesAuthorized 6,500,000,000us-gaap_CommonStockSharesAuthorized    
Preferred stock, shares authorized 20,000,000us-gaap_PreferredStockSharesAuthorized      
Purchases of treasury stock $ 7,703,000,000us-gaap_PaymentsForRepurchaseOfCommonStock $ 6,516,000,000us-gaap_PaymentsForRepurchaseOfCommonStock $ 2,591,000,000us-gaap_PaymentsForRepurchaseOfCommonStock  
Preferred stock dividend rate       5.00%us-gaap_PreferredStockDividendRatePercentage
AstraZeneca LP [Member]        
Par value preferred stock obligation assumed in connection with 1998 restructuring of AMI       2,400,000,000us-gaap_MinorityInterestAmountRepresentedByPreferredStock
/ dei_LegalEntityAxis
= us-gaap_PartnershipInterestMember
Accelerated share repurchase agreement (ASR) [Member]        
Purchases of treasury stock, shares   105,000,000us-gaap_TreasuryStockSharesAcquired
/ us-gaap_ShareRepurchaseProgramAxis
= mrk_AcceleratedsharerepurchaseagreementASRMember		    
Purchases of treasury stock   $ 5,000,000,000us-gaap_PaymentsForRepurchaseOfCommonStock
/ us-gaap_ShareRepurchaseProgramAxis
= mrk_AcceleratedsharerepurchaseagreementASRMember