Benefit Plans and Other Postretirement Benefits, Pension Plan Benefit Obligations, Cash Flows and Benefit Costs (Details) $ in Thousands		1 Months Ended	12 Months Ended
		Jan. 31, 2018 USD ($) Plan	Dec. 31, 2017 USD ($) Plan	Dec. 31, 2016 USD ($) Plan	Dec. 31, 2015 USD ($) Plan
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Settlement Charge Recognized in Operating and Maintenance Expenses			$ 205,539	$ 194,927	$ 179,413
Significant Assumptions Used to Measure Costs [Abstract]
Expected average long-term rate of return on assets (as a percent)			7.10%	7.10%
Expected average long-term rate of return on assets for next fiscal year (as a percent)			7.10%
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated Benefit Obligation at Dec. 31			$ 145,387	$ 146,448
Change in Projected Benefit Obligation [Roll Forward]
Obligation at Jan. 1		$ 156,748	157,457	152,545
Service cost			4,618	4,417	4,759
Interest cost			6,218	6,816	6,520
Plan amendments			(713)	305
Actuarial (gain) loss			6,499	7,315
Benefit payments	[1]		(17,331)	(13,941)
Obligation at Dec. 31			156,748	157,457	152,545
Defined Benefit Plan, Plan Assets, Payment for Settlement			13,000
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1		124,852	118,976	119,314
Actual return (loss) on plan assets			13,923	6,163
Employer contributions			9,284	7,440
Benefit payments	[1]		(17,331)	(13,941)
Fair value of plan assets at Dec. 31			124,852	118,976	119,314
Funded Status of Plans at Dec. 31 [Abstract]
Funded status	[2]		(31,896)	(38,481)
Amount Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]
Net loss			80,429	91,531
Prior service (credit) cost			(346)	750
Total			80,083	92,281
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]
Current regulatory assets			5,548	5,972
Noncurrent regulatory assets			74,535	86,309
Total			$ 80,083	$ 92,281
Significant Assumptions Used to Measure Benefit Obligations [Abstract]
Discount rate for year-end valuation (as a percent)			3.63%	4.13%
Mortality table			RP 2014	RP 2014
Expected average long-term increase in compensation level (as a percent)			3.75%	3.75%
Cash Flows [Abstract]
Total contributions to Xcel Energy's pension plans during the period			$ 9,000	$ 7,000	5,000
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost			4,618	4,417	4,759
Interest cost			6,218	6,816	6,520
Expected return on plan assets			(9,180)	(9,157)	(9,483)
Amortization of prior service cost (credit)			138	111	111
Amortization of net loss			5,846	5,392	6,804
Settlement charge	[3]		7,107	0	0
Net periodic pension cost			14,747	7,579	8,711
Costs not recognized due to effects of regulation			(4,176)	0	0
Net benefit cost recognized for financial reporting			10,571	$ 7,579	$ 8,711
Settlement Charge Recognized in Operating and Maintenance Expenses			$ 2,000
Significant Assumptions Used to Measure Costs [Abstract]
Discount rate (as a percent)			4.13%	4.66%	4.11%
Expected average long-term increase in compensation level (as a percent)			3.75%	4.00%	3.75%
Expected average long-term rate of return on assets (as a percent)			7.10%	7.10%	7.25%
Allocated costs for pension plans sponsored by Xcel Energy Inc.			$ 3,000	$ 2,000	$ 2,000
Expected average long-term rate of return on assets for next fiscal year (as a percent)			7.10%
Number of years fair market value of plan assets is adjusted using calculated value method (in years)			5 years
Annual adjustment rate used in calculated value method (as a percent)			20.00%
Xcel Energy Inc. | Pension Plan [Member]
Cash Flows [Abstract]
Number of pension plans to which contributions were made | Plan			4	4	4
Total contributions to Xcel Energy's pension plans during the period			$ 162,000	$ 125,000	$ 90,000
Subsequent Event | Pension Plan [Member]
Cash Flows [Abstract]
Total contributions to Xcel Energy's pension plans during the period		$ 10,000
Subsequent Event | Xcel Energy Inc. | Pension Plan [Member]
Cash Flows [Abstract]
Number of pension plans to which contributions were made | Plan		4
Total contributions to Xcel Energy's pension plans during the period		$ 150,000

[1] 2017 amount includes approximately $13 million of lump-sum benefit payments used in the determination of a settlement charge.

[2] Amounts are recognized in noncurrent liabilities on NSP-Wisconsin’s consolidated balance sheets.

[3] 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 fourth quarter of 2017 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 recorded in O&M expenses in the fourth quarter of 2017.