Pension and Other Postretirement Benefit Plans - Textual (Detail) (USD $) In Billions, unless otherwise specified	Sep. 30, 2012
Defined Benefit Plan Disclosure [Line Items]
Expected contribution to the defined benefit pension plans during 2012	$ 1.75

Defined Benefit Plan Estimated Employer Contributions In Current Fiscal Year

No definition available.

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Namespace Prefix:	mrk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant