Noncash activity affecting the restructuring reserve, including accelerated depreciation.

No definition available.

Name:	mrk_RestructuringReserveIncreaseDecreaseNoncashActivity
Namespace Prefix:	mrk_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of cash payments made as the result of exit or disposal activities. Excludes payments associated with a discontinued operation or an asset retirement obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 420
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482017/420-10-50-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 17
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-17

+ Details

Name:	us-gaap_PaymentsForRestructuring
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Discloses the amount charged against the accrued restructuring reserves, or earnings if not previously accrued, during the period for the specified type of restructuring cost.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 420
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 5.P.4.d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479823/420-10-S99-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 420
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 5.P.4.b.1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479823/420-10-S99-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 420
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482017/420-10-50-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 420
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482017/420-10-50-1

+ Details

Name:	us-gaap_RestructuringAndRelatedCostIncurredCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Carrying amount (including both current and noncurrent portions of the accrual) as of the balance sheet date pertaining to a specified type of cost associated with exit from or disposal of business activities or restructuring pursuant to a duly authorized plan.

+ References

+ Details

Name:	us-gaap_RestructuringReserve
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_RestructuringReserveRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_RestructuringPlanAxis=mrk_RestructuringProgram2025Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RestructuringPlanAxis=mrk_RestructuringProgram2024Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RestructuringCostAndReserveAxis=mrk_AcceleratedDepreciationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RestructuringCostAndReserveAxis=us-gaap_EmployeeSeveranceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RestructuringCostAndReserveAxis=us-gaap_OtherRestructuringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Restructuring - Schedule of Activities by Program (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Restructuring Reserve [Roll Forward]
Expenses	$ 390		$ 1,274
Restructuring Program, 2025
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning balance			0
Expenses	302		951
(Payments) receipts, net			(9)
Non-cash activity			(81)
Restructuring reserve, ending balance	861		861
Restructuring Program, 2024
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning balance			564
Expenses	88	$ 279	323	$ 701
(Payments) receipts, net			(239)
Non-cash activity			(149)
Restructuring reserve, ending balance	499		499
Accelerated Depreciation
Restructuring Reserve [Roll Forward]
Expenses	56		152
Accelerated Depreciation \| Restructuring Program, 2025
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning balance			0
Expenses	0		0
(Payments) receipts, net			0
Non-cash activity			0
Restructuring reserve, ending balance	0		0
Accelerated Depreciation \| Restructuring Program, 2024
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning balance			0
Expenses	56	40	152	171
(Payments) receipts, net			0
Non-cash activity			(152)
Restructuring reserve, ending balance	0		0
Separation Costs
Restructuring Reserve [Roll Forward]
Expenses	6		494
Separation Costs \| Restructuring Program, 2025
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning balance			0
Expenses	0		481
(Payments) receipts, net			(7)
Non-cash activity			0
Restructuring reserve, ending balance	474		474
Separation Costs \| Restructuring Program, 2024
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning balance			564
Expenses	6	11	13	122
(Payments) receipts, net			(78)
Non-cash activity			0
Restructuring reserve, ending balance	499		499
Other Exit Costs
Restructuring Reserve [Roll Forward]
Expenses	328		628
Other Exit Costs \| Restructuring Program, 2025
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning balance			0
Expenses	302		470
(Payments) receipts, net			(2)
Non-cash activity			(81)
Restructuring reserve, ending balance	387		387
Other Exit Costs \| Restructuring Program, 2024
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning balance			0
Expenses	26	$ 228	158	$ 408
(Payments) receipts, net			(161)
Non-cash activity			3
Restructuring reserve, ending balance	$ 0		$ 0