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Accounting Policies - Intangible Assets (Details) | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018
Marketing contract |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |
Estimated useful life (years) | 20 years | 20 years

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=dk_MarketingContractMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |