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Fair Value of Financial Assets and Liabilities, Credit Related Contingent Features (Details) - USD ($) | Dec. 31, 2018 | Dec. 31, 2017
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Fair Value Disclosures [Abstract] |  |
Derivative instruments in a gross liability position | $ 0 | $ 0
Collateral posted related to adequate assurance clauses in derivative contracts | $ 0 | $ 0

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration

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- Definition The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features for adequate assurance clauses. + References No definition available. + Details
Name: | xel_CollateralAlreadyPostedAdequateAssuranceClausesAggregateFairValue
Namespace Prefix: | xel_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a gross liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument. + References No definition available. + Details
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Namespace Prefix: | xel_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant