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Share-Based Instruments - Part 3 (Details) - Restricted Stock Units Program - DKK (kr) |  | 6 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Feb. 23, 2021 | Jun. 30, 2021 | Jun. 30, 2020 | Dec. 31, 2020 | Apr. 30, 2021 | Mar. 31, 2019 | Mar. 31, 2016
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |  |  |  |
Amount of stock repurchase plan authorized |  |  |  |  | kr 500,000 | kr 500,000 | kr 500,000
Number of shares authorized to be repurchased | 200,000 |  |  |  | 500,000 | 500,000 | 500,000
Repurchased shares |  |  | 0 | 225,000 |  |  |
Repurchased values |  |  |  | kr 225,000 |  |  |
Percentage of share capital | 0.30% |  |  |  |  |  |
Treasury stock shares |  | 299,206 |  |  |  |  |
Purchase of treasury shares | kr (447,000,000) |  |  |  |  |  |

X
- Definition Amount of stock repurchase plan authorized. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The number of shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan. + References No definition available. + Details
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Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock. + References No definition available. + Details
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Data Type: | xbrli:sharesItemType
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Period Type: | duration

X
- Definition Equity impact of the value of stock that has been repurchased during the period and has not been retired and is not held in treasury. Some state laws may mandate the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock. + References No definition available. + Details
Name: | gmab_IfrsStockRepurchasedDuringPeriodValue
Namespace Prefix: | gmab_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition The number of shares that has been treasury stock. + References No definition available. + Details
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Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of share capital. + References No definition available. + Details
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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- 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
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Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition The decrease in equity resulting from the purchase of treasury shares. [Refer: Treasury shares] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2020-01-01 -Paragraph 106 -Subparagraph d -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_106_d&doctype=Standard -URIDate 2020-01-01 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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