Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Feb. 23, 2018	Jun. 30, 2017
Document and Entity Information [Abstract]
Entity Registrant Name	NORTHERN STATES POWER CO /WI/
Entity Central Index Key	0000072909
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Document Type	10-K/A
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Amendment Flag	true
Amendment Description	We are filing this Amendment No. 1 to our Annual Report on Form 10-K/A (the "Amendment") for the fiscal year ended December 31, 2017, as filed with the Securities and Exchange Commission (the “SEC”) on February 26, 2018 (the “Original Form 10-K”), solely to correct an administrative error in the date of the Report of Independent Registered Public Accounting Firm of Deloitte & Touche LLP (the “Report”). The Report in the Original Form 10-K was incorrectly dated as “February 23, 2017.” The correct date is “February 23, 2018.” The Report of Independent Registered Public Accounting Firm with the correct date is filed herewith. In accordance with Rule 12b-15 under the Securities Exchange Act of 1934, as amended (the “Exchange Act”), we have included the entire text of Part II, Item 8 of the Original Form 10-K in this Amendment. However, there have been no changes to the text of such Part II, Item 8 other than the change stated in the immediately preceding paragraph. Further, there have been no changes to the XBRL data filed in Exhibit 101 of the Original Form 10-K. This Amendment includes new consent of Deloitte & Touche LLP as Exhibits 23.01 and new certifications pursuant to Sections 302 and 906 of the Sarbanes-Oxley Act of 2002 as Exhibits 31.01, 31.02, and 32.01 hereto. Except as expressly set forth above, this Amendment does not, and does not purport to, amend, update or restate the information in any other Item of the Original Form 10-K or reflect any events that have occurred after the filing of the Original Form 10-K.
Entity Common Stock, Shares Outstanding		933,000
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 0