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Commitments and Contingencies - Letters of Credit (Details)	Mar. 31, 2019 USD ($)
Second Amended and Restated Credit Agreement | Line of Credit | Revolving Credit Facility | Fifth Third Bank
Debt Instrument [Line Items]
Letters of credit outstanding	$ 0

X

- 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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

Name:	us-gaap_DebtInstrumentAxis=dk_SecondAmendedandRestatedCreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_LineOfCreditFacilityAxis=dk_FifthThirdBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: