Commitments and Contingencies - Sherco (Details) - USD ($) $ in Millions	1 Months Ended
Jan. 31, 2021	Jan. 27, 2021
Rate Matters [Abstract]
Customer refund of previously recovered purchased power costs	$ 17
Amount MPUC previously disallowed related to Sherco outage		$ 22

Customer refund of previously recovered purchased power costs

No definition available.

Name:	nspm_Customerrefundofpreviouslyrecoveredpurchasedpowercosts
Namespace Prefix:	nspm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration