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Benefit Plans and Other Postretirement Benefits (Tables)
 12 Months Ended
Dec. 31, 2021
Benefit Plans and Other Postretirement Benefits [Abstract]  
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost
Pension BenefitsPostretirement Benefits
(Millions of Dollars)2021202020212020
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost:
Net loss$307 $414 $31 $37 
Prior service credit— — (4)(6)
Total$307 $414 $27 $31 
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates:
Current regulatory assets$25 $29 $— $— 
Noncurrent regulatory assets282 385 25 29 
Deferred income taxes— — 1 1 
Net-of-tax accumulated other comprehensive income— — 1 1 
Total$307 $414 $27 $31 
Measurement dateDec 31, 2021Dec 31, 2020Dec 31, 2021Dec 31, 2020
Projected Benefit Payments for the Pension and Postretirement Benefit Plans
Projected Benefit Payments
NSP-Minnesota’s projected benefit payments:
(Millions of Dollars)Projected
Pension Benefit
Payments		Gross Projected
Postretirement
Health Care
Benefit Payments		Expected
Medicare Part D
Subsidies		Net Projected
Postretirement
Health Care
Benefit Payments
2022$118 $6 $— $6 
202372 6 — 6 
202468 5 — 5 
202567 5 — 5 
202664 5 — 5 
2027-2031292 18 — 18 
Pension Plan  
Benefit Plans and Other Postretirement Benefits [Abstract]  
Target Asset Allocations and Plan Assets Measured at Fair Value or each of the fair value hierarchy levels, NSP-Minnesota’s pension plan assets measured at fair value:
Dec. 31, 2021 (a)
Dec. 31, 2020 (a)
(Millions of Dollars)Level 1Level 2Level 3Measured at NAVTotalLevel 1Level 2Level 3Measured at NAVTotal
Cash equivalents$31 $— $— $— $31 $52 $— $— $— $52 
Commingled funds304 — — 274 578 369 — — 284 653 
Debt securities— 219 1 — 220 — 167 1 — 168 
Equity securities16 — — — 16 20 — — — 20 
Other— 1 — 7 8 3 1 — — 4 
Total$351 $220 $1 $281 $853 $444 $168 $1 $284 $897 
(a)See Note 8 for further information on fair value measurement inputs and methods.
For each of the fair value hierarchy levels, NSP-Minnesota’s postretirement benefit plan assets that were measured at fair value:
Dec. 31, 2021 (a)
Dec. 31, 2020 (a)
(Millions of Dollars)Level 1Level 2Level 3Measured at NAVTotalLevel 1Level 2Level 3Measured at NAVTotal
Commingled funds$— $— $— $1 $1 $— $— $— $— $— 
Debt securities— 2 — — 2 — 2 — — 2 
Total$— $2 $— $1 $3 $— $2 $— $— $2 
Changes in Projected Benefit Obligations, Fair Value of Plan Assets, and Funded Status of Plan [Table Text Block]
Pension BenefitsPostretirement Benefits
(Millions of Dollars)2021202020212020
Change in Benefit Obligation:
Obligation at Jan. 1$989 $942 $73 $76 
Service cost30 27 — — 
Interest cost25 31 2 2 
Plan amendments1 — — — 
Actuarial (gain) loss(28)84 (5)2 
Benefit payments(140)(95)(6)(7)
Obligation at Dec. 31$877 $989 $64 $73 
Change in Fair Value of Plan Assets:
Fair value of plan assets at Jan. 1$897 $815 $2 $3 
Actual return on plan assets62 133 — — 
Employer contributions34 44 7 6 
Benefit payments(140)(95)(6)(7)
Fair value of plan assets at Dec. 31$853 $897 $3 $2 
Funded status of plans at Dec. 31$(24)$(92)$(61)$(71)
Amounts recognized in the Consolidated Balance Sheet at Dec. 31:
Current liabilities$— $— $(3)$(5)
Noncurrent liabilities(24)(92)(58)(66)
Net amounts recognized$(24)$(92)$(61)$(71)
    
Pension BenefitsPostretirement Benefits
Significant Assumptions Used to Measure Benefit Obligations:2021202020212020
Discount rate for year-end valuation3.08 %2.71 %3.09 %2.65 %
Expected average long-term increase in compensation level3.75 %3.75 %N/AN/A
Mortality tablePri-2012Pri-2012Pri-2012Pri-2012
Health care costs trend rate — initial: Pre-65N/AN/A5.30 %5.50 %
Health care costs trend rate — initial: Post-65N/AN/A4.90 %5.00 %
Ultimate trend assumption — initial: Pre-65N/AN/A4.50 %4.50 %
Ultimate trend assumption — initial: Post-65N/AN/A4.50 %4.50 %
Years until ultimate trend is reachedN/AN/A45
Components of Net Periodic Benefit Costs
Pension BenefitsPostretirement Benefits
(Millions of Dollars)202120202019202120202019
Service cost$30 $27 $25 $— $— $— 
Interest cost25 31 37 2 2 3 
Expected return on plan assets(52)(55)(54)— — — 
Amortization of prior service cost— — — (3)(3)(3)
Amortization of net loss34 33 30 2 1 2 
Settlement charge (a)
35 — — — — — 
Net periodic pension cost72 36 38 1 — 2 
Effects of regulation(44)(4)(5)— — — 
Net benefit cost recognized for financial reporting$28 $32 $33 $1 $— $2 
Significant Assumptions Used to Measure Costs:
Discount rate2.71 %3.49 %4.31 %2.65 %3.47 %4.32 %
Expected average long-term increase in compensation level3.75 3.75 3.75 — — — 
Expected average long-term rate of return on assets6.60 7.10 7.10 4.10 4.50 4.50 
(a)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 2021, as a result of lump-sum distributions during the 2021 plan year, NSP-Minnesota recorded a total pension settlement charge of $35 million in 2021, which was not recognized due to the effects of regulation. There were no settlement charges recorded to the qualified pension plans in 2020 and 2019.
Postretirement Benefits Plan  
Benefit Plans and Other Postretirement Benefits [Abstract]  
Target Asset Allocations and Plan Assets Measured at Fair Value
Target asset allocations:
Pension BenefitsPostretirement Benefits
2021202020212020
Domestic and international equity securities33 %35 %15 %15 %
Long-duration fixed income and interest rate swap securities37 35 — — 
Short-to-intermediate fixed income securities11 13 71 72 
Alternative investments17 15 8 9 
Cash2 2 6 4 
Total100 %100 %100 %100 %
The asset allocations above reflect target allocations approved in the calendar year to take effect in the subsequent year