OTHER DISCLOSURES - Fees to Auditors (Details) - DKK (kr) kr in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fees to Auditors Appointed at the Annual General Meeting
Audit services	kr 1.9	kr 1.1	kr 1.1
Audit-related services	2.3	0.1	0.4
Tax and VAT services	0.5	0.4	0.7
Other services	2.4	0.1
Total	7.1	1.7	2.2
Fees for other services	kr 5.2	kr 0.6	kr 1.1

The amount of fees paid or payable to the entity's auditors for audit related services.

No definition available.

Name:	gmab_AuditorSRemunerationForAuditRelatedServices
Namespace Prefix:	gmab_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of fees paid or payable to the entity's auditors for other services.

+ References

No definition available.

+ Details

Name:	gmab_FeesForOtherAuditServices
Namespace Prefix:	gmab_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of fees paid or payable to the entity's auditors.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_112_c&doctype=Standard
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_AuditorsRemuneration
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- References

No definition available.

+ Details

Name:	ifrs-full_AuditorsRemunerationAbstract
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of fees paid or payable to the entity's auditors for auditing services.

+ References

+ Details

Name:	ifrs-full_AuditorsRemunerationForAuditServices
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of fees paid or payable to the entity's auditors for services that the entity does not separately disclose in the same statement or note.

+ References

+ Details

Name:	ifrs-full_AuditorsRemunerationForOtherServices
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of fees paid or payable to the entity's auditors for tax services.

+ References

+ Details

Name:	ifrs-full_AuditorsRemunerationForTaxServices
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration