The amount of collaboration agreements, development regulatory and commercial milestone payment.

No definition available.

Name:	gmab_CollaborationAgreementsDevelopmentRegulatoryAndCommercialMilestonePayment
Namespace Prefix:	gmab_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of upfront payment under collaboration agreement term.

+ References

No definition available.

+ Details

Name:	gmab_CollaborationAgreementsUpfrontPayment
Namespace Prefix:	gmab_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	gmab_CollaborativeArrangementAndArrangementOtherThanCollaborativeLineItems
Namespace Prefix:	gmab_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

License exercisable term.

+ References

No definition available.

+ Details

Name:	gmab_LicenseExercisableTerm
Namespace Prefix:	gmab_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of additional proprietary targets of license.

+ References

No definition available.

+ Details

Name:	gmab_NumberOfAdditionalProprietaryTargetsOfLicense
Namespace Prefix:	gmab_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of options to obtain commercial licenses.

+ References

No definition available.

+ Details

Name:	gmab_NumberOfOptionsToObtainCommercialLicenses
Namespace Prefix:	gmab_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of proprietary targets of license.

+ References

No definition available.

+ Details

Name:	gmab_NumberOfProprietaryTargetsOfLicense
Namespace Prefix:	gmab_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of identifiable non-monetary assets without physical substance. This amount does not include goodwill. [Refer: Goodwill]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 38
-IssueDate 2019-01-01
-Paragraph 118
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=38&code=ifrs-tx-2019-en-r&anchor=para_118_e&doctype=Standard
-URIDate 2019-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 54
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_54_c&doctype=Standard
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_IntangibleAssetsOtherThanGoodwill
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of intangible assets assessed as having an indefinite useful life. [Refer: Intangible assets other than goodwill]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 38
-IssueDate 2019-01-01
-Paragraph 122
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=38&code=ifrs-tx-2019-en-r&anchor=para_122_a&doctype=Standard
-URIDate 2019-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 36
-IssueDate 2019-01-01
-Paragraph 134
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=36&code=ifrs-tx-2019-en-r&anchor=para_134_b&doctype=Standard
-URIDate 2019-03-27

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 36
-IssueDate 2019-01-01
-Paragraph 135
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=36&code=ifrs-tx-2019-en-r&anchor=para_135_b&doctype=Standard
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_IntangibleAssetsWithIndefiniteUsefulLife
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of investments accounted for using the equity method. The equity method is a method of accounting whereby the investment is initially recognised at cost and adjusted thereafter for the post-acquisition change in the investor's share of net assets of the investee. The investor's profit or loss includes its share of the profit or loss of the investee. The investor's other comprehensive income includes its share of the other comprehensive income of the investee. [Refer: At cost [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2019-01-01
-Paragraph 24
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=8&code=ifrs-tx-2019-en-r&anchor=para_24_a&doctype=Standard
-URIDate 2019-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 12
-IssueDate 2019-01-01
-Paragraph B16
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=12&code=ifrs-tx-2019-en-r&anchor=para_B16&doctype=Appendix&subtype=B
-URIDate 2019-03-27

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 54
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_54_e&doctype=Standard
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_InvestmentAccountedForUsingEquityMethod
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	gmab_IfrsCollaborativeArrangementAndArrangementOtherThanCollaborativeAxis=gmab_CurevacCollaborationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	gmab_IfrsCollaborativeArrangementAndArrangementOtherThanCollaborativeAxis=gmab_ImmaticsBiotechnologiesGmbhMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_BottomOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_TopOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis=ifrs-full_LicencesAndFranchisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis=ifrs-full_GrossCarryingAmountMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

OPERATING ASSETS AND LIABILITIES - Research collaboration and license agreement (Details) € in Millions, kr in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019 EUR (€)	Dec. 31, 2019 Option	Dec. 31, 2019 USD ($)	Dec. 31, 2019 item	Dec. 31, 2019 DKK (kr)	Dec. 19, 2019 DKK (kr)	Dec. 31, 2018 DKK (kr)	Jul. 31, 2018 property	Jul. 31, 2018 USD ($)	Jul. 31, 2018 item	Dec. 31, 2017 DKK (kr)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Other investments \| kr					kr 149
Carrying amount of intangible asset \| kr					470		kr 470
CureVac
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront Payment \| $			$ 10
Other investments	€ 20				149	kr 149
Number of options		3		3
License exercisable term	5 years
Collaborative arrangement, Development, Regulatory and commercial milestone payment \| $			280
Immatics
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of proprietary targets of license								3		3
Number of additional proprietary targets of license \| item										2
Upfront Payment \| $									$ 54
Collaborative arrangement, Development, Regulatory and commercial milestone payment \| $									$ 550
Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaborative arrangement, Development, Regulatory and commercial milestone payment \| $			275
Minimum \| CureVac
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaborative arrangement, Development, Regulatory and commercial milestone payment \| $			275
Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaborative arrangement, Development, Regulatory and commercial milestone payment \| $			368
Maximum \| CureVac
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaborative arrangement, Development, Regulatory and commercial milestone payment \| $			$ 368
Licences, Rights and Patents
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Carrying amount of intangible asset \| kr					470		470
Licences, Rights and Patents \| CureVac
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Carrying amount of intangible asset \| kr					67
Cost
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Carrying amount of intangible asset \| kr					897		798				kr 392
Cost \| Immatics
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Carrying amount of intangible asset \| kr					274		323
Previously acquired licenses in use \| kr					152
Cost \| Licences, Rights and Patents
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Carrying amount of intangible asset \| kr					kr 897		kr 798				kr 392