OPERATING ASSETS AND LIABILITIES - Other Payables (Details) - DKK (kr) kr in Millions	Dec. 31, 2019	Dec. 31, 2018
OPERATING ASSETS AND LIABILITIES
Liabilities related to collaboration agreements	kr 8	kr 6
Staff cost liabilities	48	30
Other liabilities	715	213
Accounts payable	69	69
Total at December 31	840	318
Non-current other payables	1	2
Current other payables	839	kr 316
Pension obligations	kr 0

The amount of liabilities under collaboration agreement.

No definition available.

Name:	gmab_CollaborationAgreementsLiabilities
Namespace Prefix:	gmab_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of pension obligation under defined contribution plan recognised.

+ References

No definition available.

+ Details

Name:	gmab_DefinedContributionPlansPensionObligation
Namespace Prefix:	gmab_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

The amount of current payables that the entity does not separately disclose in the same statement or note.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 55
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_55&doctype=Standard
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_OtherCurrentPayables
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of liabilities that the entity does not separately disclose in the same statement or note.

+ References

+ Details

Name:	ifrs-full_OtherLiabilities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of non-current payables that the entity does not separately disclose in the same statement or note.

+ References

+ Details

Name:	ifrs-full_OtherNoncurrentPayables
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amounts payable that the entity does not separately disclose in the same statement or note.

+ References

+ Details

Name:	ifrs-full_OtherPayables
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of provisions for employee benefits. [Refer: Employee benefits expense; Provisions]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 78
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_78_d&doctype=Standard
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_ProvisionsForEmployeeBenefits
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of payment due to suppliers for goods and services used in the entity's business.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 78
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_78&doctype=Standard
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_TradeAndOtherPayablesToTradeSuppliers
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant