Credit Related Contingent Features (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Derivative instruments in a gross liability position	$ 7.1	$ 0.0
Collateral posted related to adequate assurance clauses in derivative contracts	$ 0.0	$ 0.0

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