Related-Party Transactions (Details) - EBP 001 - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024
Trustee
EBP, Related Party and Party-in-Interest Transactions [Line Items]
Investment in Master Trust, at fair value	$ 794.1	$ 648.3
Increase from dividend income on investment	10.5
Realized gain on investment	21.9
Unrealized gain on investment	197.8
Merck common stock
EBP, Related Party and Party-in-Interest Transactions [Line Items]
Investment in Master Trust, at fair value	812.0	$ 926.9
Increase from dividend income on investment	28.0
Realized gain on investment	77.6
Unrealized gain on investment	53.3
Purchase price	28.7
Sale of common stock	$ 162.6

Amount of increase (decrease) in net asset available for benefit from realized gain (loss) on investment held by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

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Amount of purchase price for plan interest in master trust acquired by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

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Fair value of investment, excluding plan interest in master trust, held by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Includes, but is not limited to, derivative, investment measured at net asset value per share as practical expedient, and insurance contract not meeting fully-benefit responsive investment criteria.

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Amount of selling price for plan interest in master trust sold by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

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