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Income Taxes Federal Audits (Details) $ in Millions	Mar. 31, 2021 USD ($)
Income Tax Disclosure [Abstract]
State income tax adjustment	$ 0

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- Definition

State income tax adjustment

+ References

No definition available.

+ Details

Name:	nspm_StateIncomeTaxAdjustment
Namespace Prefix:	nspm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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- References

No definition available.

+ Details

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