Pension and Other Postretirement Benefit Plans - Summary of Changes in Fair Value of Company's Level 3 Pension Plan Assets (Details) - Pension Benefits - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	$ 860
Actual return on plan assets:
Fair value of plan assets December 31	942	$ 860
United States
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	11,361	9,648
Actual return on plan assets:
Fair value of plan assets December 31	12,672	11,361
United States \| Other
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	9
Actual return on plan assets:
Fair value of plan assets December 31	7	9
United States \| Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	9	13
Actual return on plan assets:
Relating to assets still held at December 31	(5)	(8)
Relating to assets sold during the year	5	8
Purchases and sales, net	(2)	(4)
Fair value of plan assets December 31	7	9
United States \| Level 3 \| Insurance Contracts
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	0	0
Actual return on plan assets:
Relating to assets still held at December 31	0	0
Relating to assets sold during the year	0	0
Purchases and sales, net	0	0
Fair value of plan assets December 31	0	0
United States \| Level 3 \| Real Estate
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	0	0
Actual return on plan assets:
Relating to assets still held at December 31	0	0
Relating to assets sold during the year	0	0
Purchases and sales, net	0	0
Fair value of plan assets December 31	0	0
United States \| Level 3 \| Other
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	9	13
Actual return on plan assets:
Relating to assets still held at December 31	(5)	(8)
Relating to assets sold during the year	5	8
Purchases and sales, net	(2)	(4)
Fair value of plan assets December 31	7	9
International
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	10,163	8,580
Actual return on plan assets:
Fair value of plan assets December 31	12,049	10,163
International \| Insurance Contracts
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	916
Actual return on plan assets:
Fair value of plan assets December 31	1,012	916
International \| Real Estate
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	1
Actual return on plan assets:
Fair value of plan assets December 31	16	1
International \| Other
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	21
Actual return on plan assets:
Fair value of plan assets December 31	6	21
International \| Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	851	813
Actual return on plan assets:
Relating to assets still held at December 31	103	54
Purchases and sales, net	(17)	(16)
Transfers out of Level 3	(2)	0
Fair value of plan assets December 31	935	851
International \| Level 3 \| Insurance Contracts
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	851	811
Actual return on plan assets:
Relating to assets still held at December 31	103	54
Purchases and sales, net	(17)	(14)
Transfers out of Level 3	(2)	0
Fair value of plan assets December 31	935	851
International \| Level 3 \| Real Estate
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	0	1
Actual return on plan assets:
Relating to assets still held at December 31	0	0
Purchases and sales, net	0	(1)
Transfers out of Level 3	0	0
Fair value of plan assets December 31	0	0
International \| Level 3 \| Other
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	0	1
Actual return on plan assets:
Relating to assets still held at December 31	0	0
Purchases and sales, net	0	(1)
Transfers out of Level 3	0	0
Fair value of plan assets December 31	$ 0	$ 0

No definition available.

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

- Definition

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets sold.

+ References

+ Details

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

- Definition

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets still held.

+ References

+ Details

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

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

- Definition

Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(01)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(01)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(ii)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(02)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 7: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(ii)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, measured using unobservable input, of increase (decrease) in plan asset of defined benefit plan from purchase, sale and settlement of trade associated with underlying investment.

+ References

+ Details

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

- Definition

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan, for assets transferred into (out of) Level 3 of fair value hierarchy. Includes, but is not limited to, transfer due to change in observability of significant inputs.

+ References

+ Details

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

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanSponsorLocationAxis=country_US
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_OtherInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=mrk_InsuranceContractsOtherInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=mrk_RealEstateInvestmentFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanSponsorLocationAxis=us-gaap_ForeignPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: