Name:	gmab_AntidilutiveWarrantsExcludedFromComputationOfEarningsPerShareAmount
Namespace Prefix:	gmab_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

Warrants vested that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

No definition available.

Name:	gmab_AntidilutiveWarrantsVestedExcludedFromComputationOfEarningsPerShareAmount
Namespace Prefix:	gmab_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

The number of ordinary shares excluding shares outstanding.

No definition available.

Name:	gmab_AverageNumberOfOrdinarySharesExcludingOutstanding
Namespace Prefix:	gmab_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

The number of treasury shares outstanding.

No definition available.

Name:	gmab_AverageNumberOfTreasurySharesOutstanding
Namespace Prefix:	gmab_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

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

The weighted average number of ordinary shares outstanding plus the weighted average number of ordinary shares that would be issued on the conversion of all the dilutive potential ordinary shares into ordinary shares. [Refer: Ordinary shares [member]; Weighted average [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 33
-IssueDate 2019-01-01
-Paragraph 70
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=33&code=ifrs-tx-2019-en-r&anchor=para_70_b&doctype=Standard
-URIDate 2019-03-27

Name:	ifrs-full_AdjustedWeightedAverageShares
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

The amount of profit (loss) attributable to ordinary equity holders of the parent entity (the numerator) divided by the weighted average number of ordinary shares outstanding during the period (the denominator).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 33
-IssueDate 2019-01-01
-Paragraph 66
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=33&code=ifrs-tx-2019-en-r&anchor=para_66&doctype=Standard
-URIDate 2019-03-27

Name:	ifrs-full_BasicEarningsLossPerShare
Namespace Prefix:	ifrs-full_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

The amount of profit (loss) attributable to ordinary equity holders of the parent entity (the numerator), divided by the weighted average number of ordinary shares outstanding during the period (the denominator), both adjusted for the effects of all dilutive potential ordinary shares. [Refer: Ordinary shares [member]; Weighted average [member]]

Name:	ifrs-full_DilutedEarningsLossPerShare
Namespace Prefix:	ifrs-full_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

The number of dilutive potential ordinary shares that relate to the assumed conversion of the entity's convertible instruments.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 33
-IssueDate 2019-01-01
-Paragraph 70
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=33&code=ifrs-tx-2019-en-r&anchor=para_70_b&doctype=Standard
-URIDate 2019-03-27

Name:	ifrs-full_DilutiveEffectOfConvertibleInstrumentsOnNumberOfOrdinaryShares
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

The total of income less expenses from continuing and discontinued operations, excluding the components of other comprehensive income. [Refer: Other comprehensive income]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 7
-IssueDate 2019-01-01
-Paragraph 18
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=7&code=ifrs-tx-2019-en-r&anchor=para_18_b&doctype=Standard
-URIDate 2019-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 1
-IssueDate 2019-01-01
-Paragraph 24
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=1&code=ifrs-tx-2019-en-r&anchor=para_24_b&doctype=Standard
-URIDate 2019-03-27

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 1
-IssueDate 2019-01-01
-Paragraph 32
-Subparagraph a
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=1&code=ifrs-tx-2019-en-r&anchor=para_32_a_ii&doctype=Standard
-URIDate 2019-03-27

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2019-01-01
-Paragraph 28
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=8&code=ifrs-tx-2019-en-r&anchor=para_28_b&doctype=Standard
-URIDate 2019-03-27

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Note Effective on first application of IFRS 9
-Name IFRS
-Number 4
-IssueDate 2019-01-01
-Paragraph 39L
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=4&code=ifrs-tx-2019-en-b&anchor=para_39L_e&doctype=Standard
-URIDate 2019-03-27

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2019-01-01
-Paragraph 23
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=8&code=ifrs-tx-2019-en-r&anchor=para_23&doctype=Standard
-URIDate 2019-03-27

Reference 7: http://www.xbrl.org/2003/role/exampleRef
-Name IFRS
-Number 12
-IssueDate 2019-01-01
-Paragraph B10
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=12&code=ifrs-tx-2019-en-r&anchor=para_B10_b&doctype=Appendix&subtype=B
-URIDate 2019-03-27

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 106
-Subparagraph d
-Clause i
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_106_d_i&doctype=Standard
-URIDate 2019-03-27

Reference 9: http://www.xbrl.org/2003/role/exampleRef
-Note Effective 2021-01-01
-Name IFRS
-Number 17
-IssueDate 2019-01-01
-Paragraph 113
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=17&code=ifrs-tx-2019-en-r&anchor=para_113_b&doctype=Standard
-URIDate 2019-03-27

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 81A
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_81A_a&doctype=Standard
-URIDate 2019-03-27

Name:	ifrs-full_ProfitLoss
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

The number of ordinary shares outstanding at the beginning of the period, adjusted by the number of ordinary shares bought back or issued during the period multiplied by a time-weighting factor.