Basis of Presentation (Policies)	3 Months Ended
Basis of Presentation (Policies)	Mar. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of PresentationThe accompanying unaudited condensed consolidated financial statements of Merck & Co., Inc. (Merck or the Company) have been prepared pursuant to the rules and regulations for reporting on Form 10-Q. Accordingly, certain information and disclosures required by accounting principles generally accepted in the United States (U.S.) (GAAP) for complete consolidated financial statements are not included herein. These interim statements should be read in conjunction with the audited financial statements and notes thereto included in Merck’s Form 10-K filed on February 25, 2022.The results of operations of any interim period are not necessarily indicative of the results of operations for the full year. In the Company’s opinion, all adjustments necessary for a fair statement of these interim statements have been included and are of a normal and recurring nature.
Reclassifications	Reclassifications — Certain reclassifications have been made to prior year amounts to conform to the current year presentation
Recently Adopted Accounting Standards and Recently Issued Accounting Standards Not Yet Adopted	Recently Adopted Accounting Standards In August 2020, the Financial Accounting Standards Board (FASB) issued amended guidance on the accounting for convertible instruments and contracts in an entity’s own equity. The guidance removes the separation model for convertible debt instruments and preferred stock, amends requirements for conversion options to be classified in equity as well as amends diluted earnings per share (EPS) calculations for certain convertible debt instruments. The Company adopted the new guidance on January 1, 2022 using a modified retrospective approach. There was no impact to the Company’s consolidated financial statements upon adoption. In November 2021, the FASB issued new guidance to increase the transparency of transactions with a government that are accounted for by applying a grant or contribution accounting model by analogy. The guidance requires annual disclosures of such transactions to include the nature of the transactions and the significant terms and conditions, the accounting treatment and the impact to a company’s financial statements. The Company adopted the new guidance on January 1, 2022 on a prospective basis. There was no material impact to the Company’s consolidated financial statements upon adoption. Recently Issued Accounting Standards Not Yet Adopted In March 2020, the FASB issued optional guidance to ease the potential burden in accounting for (or recognizing the effects of) reference rate reform on financial reporting and subsequently issued clarifying amendments. The guidance provides optional expedients and exceptions for accounting for contracts, hedging relationships, and other transactions that reference the London Interbank Offered Rate (LIBOR) or another reference rate expected to be discontinued because of reference rate reform. The optional guidance is effective upon issuance and can be applied on a prospective basis at any time between January 1, 2020 through December 31, 2022. The Company is currently evaluating the impact of adoption on its consolidated financial statements. The Company is progressing in its evaluation of LIBOR cessation exposures, including the review of debt-related contracts, leases, business development and licensing arrangements, royalty and other agreements. The Company has amended certain agreements and continues to review other agreements for potential impacts. With regard to debt-related exposures in particular, the Company’s four remaining interest rate swaps linked to LIBOR will mature in September 2022. The Company will transition its LIBOR-based debt to an alternative reference rate upon LIBOR discontinuance. Based on its evaluation thus far, the Company does not anticipate a material impact to its consolidated financial statements as a result of reference rate reform. In October 2021, the FASB issued amended guidance that requires acquiring entities to recognize and measure contract assets and liabilities in a business combination in accordance with existing revenue recognition guidance. The amended guidance is effective for interim and annual periods in 2023 and is to be applied prospectively. Early adoption is permitted on a retrospective basis to the beginning of the fiscal year of adoption. The adoption of this guidance will not have an impact on the Company’s consolidated financial statements for prior acquisitions; however, the impact in future periods will be dependent upon the contract assets and contract liabilities acquired in future business combinations.
Legal Costs	Legal defense costs expected to be incurred in connection with a loss contingency are accrued when probable and reasonably estimable.