Master Trust - Schedule of Master Trust Net Assets (Details) - EBP 001 - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
EBP, Master Trust [Line Items]
Net assets available for benefits	$ 16,132,479	$ 14,970,917
Master Trust
EBP, Master Trust [Line Items]
Accrued interest and dividends	15,877	17,180
Other net assets	8,648	4,994
Net assets available for benefits	16,881,531	15,655,880
Master Trust \| Registered investment companies (mutual funds)
EBP, Master Trust [Line Items]
Investments, at fair value	97,747	500,106
Master Trust \| Common/collective trusts
EBP, Master Trust [Line Items]
Investments, at fair value	12,677,091	11,107,463
Master Trust \| Equity securities
EBP, Master Trust [Line Items]
Investments, at fair value	2,307,397	2,244,340
Master Trust \| Fixed income securities
EBP, Master Trust [Line Items]
Investments, at fair value	406,994	386,182
Master Trust \| Self-directed brokerage accounts
EBP, Master Trust [Line Items]
Investments, at fair value	459,800	368,784
Master Trust \| Merck common stock
EBP, Master Trust [Line Items]
Investments, at fair value	907,977	1,026,831
Merck US Savings Plan Interest
EBP, Master Trust [Line Items]
Accrued interest and dividends	14,659	15,934
Other net assets	8,233	4,747
Net assets available for benefits	16,046,971	14,892,909
Merck US Savings Plan Interest \| Registered investment companies (mutual funds)
EBP, Master Trust [Line Items]
Investments, at fair value	91,946	474,904
Merck US Savings Plan Interest \| Common/collective trusts
EBP, Master Trust [Line Items]
Investments, at fair value	12,108,089	10,620,120
Merck US Savings Plan Interest \| Equity securities
EBP, Master Trust [Line Items]
Investments, at fair value	2,189,256	2,130,066
Merck US Savings Plan Interest \| Fixed income securities
EBP, Master Trust [Line Items]
Investments, at fair value	386,758	367,820
Merck US Savings Plan Interest \| Self-directed brokerage accounts
EBP, Master Trust [Line Items]
Investments, at fair value	435,989	352,416
Merck US Savings Plan Interest \| Merck common stock
EBP, Master Trust [Line Items]
Investments, at fair value	$ 812,041	$ 926,902

Fair value of investment 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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