Benefit Plans and Other Postretirement Benefits (Details) $ in Thousands		1 Months Ended	3 Months Ended				9 Months Ended				12 Months Ended
		Jan. 31, 2018 USD ($) Plan	Sep. 30, 2018 USD ($)		Sep. 30, 2017 USD ($)		Sep. 30, 2018 USD ($)		Sep. 30, 2017 USD ($)		Dec. 31, 2017 USD ($)
Components of Net Periodic Benefit Cost [Abstract]
Operating and maintenance expenses			$ 50,057		$ 51,130		$ 149,335		$ 148,846
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement			5,181	[1]	0	[1]	5,181	[1]	0	[1]	$ 2,000
Components of Net Periodic Benefit Cost [Abstract]
Service cost			1,194		1,154		3,583		3,463
Interest cost	[2]		1,361		1,554		4,081		4,663
Expected return on plan assets	[2]		(2,256)		(2,295)		(6,769)		(6,885)
Amortization of prior service cost (credit)	[2]		(8)		35		(22)		104
Amortization of net loss	[2]		1,418		1,462		4,255		4,385
Defined Benefit Plan Net Benefit Cost (Credit) Recognized Before Regulatory Adjustments			6,890		1,910		10,309		5,730
Defined Benefit Plan Credits Not Recognized Due To Effects of Regulation			(3,053)		0		(2,832)		0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)			3,837		1,910		7,477		5,730
Operating and maintenance expenses							1,000				$ 7,000
Total contributions to the pension plans during the period		$ 10,000
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	[1]		0		0		0		0
Components of Net Periodic Benefit Cost [Abstract]
Service cost			9		7		27		21
Interest cost	[2]		142		147		426		443
Expected return on plan assets	[2]		(15)		(7)		(47)		(23)
Amortization of prior service cost (credit)	[2]		(88)		(88)		(264)		(264)
Amortization of net loss	[2]		138		109		416		327
Defined Benefit Plan Net Benefit Cost (Credit) Recognized Before Regulatory Adjustments			186		168		558		504
Defined Benefit Plan Credits Not Recognized Due To Effects of Regulation			0		0		0		0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)			$ 186		$ 168		$ 558		$ 504
Xcel Energy Inc. | Pension Plan [Member]
Components of Net Periodic Benefit Cost [Abstract]
Total contributions to the pension plans during the period		$ 150,000
Number of Xcel Energy's pension plans to which contributions were made | Plan		4

[1] A settlement charge is required when the amount of lump-sum distributions during the year is greater than the sum of the service and interest cost components of the annual net periodic pension cost. In the third quarter as a result of lump-sum distributions during the 2018 plan year, NSP-Wisconsin recorded a pension settlement charge of $5 million, the majority of which was not recognized due to the effects of regulation. A total of $1 million of that amount was recorded in other expense in the third quarter of 2018. In the fourth quarter of 2018 as a result of lump-sum distributions during the 2017 plan year, NSP-Wisconsin recorded a total pension settlement charge of $7 million, the majority of which was not recognized due to the effects of regulation. A total of $2 million of that amount was expensed in the fourth quarter of 2017.

[2] The components of net periodic cost other than the service cost component are included in the line item “other expense, net” in the income statement or capitalized on the balance sheet as a regulatory asset.