Fair Value of Financial Assets and Liabilities - Credit Related Contingent Features (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Derivative instruments in a gross liability position	$ 4	$ 4
Collateral posted related to adequate assurance clauses in derivative contracts	0	0
Derivative, Gross Liability with Cross Default Position, Aggregate Fair Value	$ 29	$ 14

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.

No definition available.

Name:	nspm_DerivativeGrossLiabilityPositionAggregateFairValue
Namespace Prefix:	nspm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant