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Commitments and Contingencies, Other Commitments (Details) | 12 Months Ended
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Dec. 31, 2017
Accenture Agreement [Member] |
Other Commitments [Line Items] |
Technology Agreement Expiration Date | 2020

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

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- Definition Technology Agreement Expiration + References No definition available. + Details
Name: | xel_TechnologyAgreementExpiration
Namespace Prefix: | xel_
Data Type: | us-types:durationStringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=xel_AccentureAgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |