Share-Based Instruments - Part 2 (Details) - DKK (kr) kr / shares in Units, kr in Millions	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020
Disclosure of terms and conditions of share-based payment arrangement [line items]
Granted	147,568	49,323
Exercised	74,992	272,078
Weighted average exercise price of warrants granted	kr 2,189.21	kr 1,548.22
Weighted average exercise price of fair market value	683.79	465.70
Weighted average exercise price of warrants exercised	kr 606.22	kr 261.06
Exercise of warrants	kr 46	kr 54
Percentage of share capital attributed to exercise of warrants	0.11%	0.42%
Share-based compensation expenses	kr 142	kr 98
Warrants [Member]
Disclosure of terms and conditions of share-based payment arrangement [line items]
Exercise of warrants	kr 46	kr 71

The number of warrants exercised during period.

No definition available.

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

- Definition

The number of warrants granted during period.

+ References

No definition available.

+ Details

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

- Definition

Percentage of share capital attributed by exercise of warrants which increase the share capital.

+ References

No definition available.

+ Details

Name:	gmab_PercentageOfShareCapitalAttributedToExerciseOfWarrants
Namespace Prefix:	gmab_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash inflow from the exercise of warrants.

+ References

No definition available.

+ Details

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

- Definition

Exercise price per share or per unit of fair value of warrants during the period.

+ References

No definition available.

+ Details

Name:	gmab_WeightedAverageExercisePriceOfFairValueOfWarrants
Namespace Prefix:	gmab_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Exercise price per share or per unit of warrants exercised during the period.

+ References

No definition available.

+ Details

Name:	gmab_WeightedAverageExercisePriceOfWarrantsExercised
Namespace Prefix:	gmab_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Exercise price per share or per unit of warrants granted during the period.

+ References

No definition available.

+ Details

Name:	gmab_WeightedAverageExercisePriceOfWarrantsGranted
Namespace Prefix:	gmab_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

The amount of expense arising from share-based payment transactions in which the goods or services received did not qualify for recognition as assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 2
-IssueDate 2020-01-01
-Paragraph 51
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=2&code=ifrs-tx-2020-en-r&anchor=para_51_a&doctype=Standard
-URIDate 2020-01-01

+ Details

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

- Details

Name:	ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=gmab_WarrantsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: