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Income Taxes (Tables)
 12 Months Ended
Dec. 31, 2020
Income Tax Disclosure [Abstract]  
Summary of Statute of Limitations Applicable to Open Tax Years
Tax Year(s)Expiration
2014 - 2016July 2021
Reconciliation of Unrecognized Tax Benefits
Unrecognized tax benefits — permanent vs temporary:
(Millions of Dollars)Dec. 31, 2020Dec. 31, 2019
Unrecognized tax benefit — Permanent tax positions$3 $3 
Unrecognized tax benefit — Temporary tax positions4 2 
Total unrecognized tax benefit$7 $5 
Changes in unrecognized tax benefits:
(Millions of Dollars)202020192018
Balance at Jan. 1$5 $5 $4 
Additions based on tax positions related to the current year1 — 1 
Additions for tax positions of prior years5 — — 
Reductions for tax positions of prior years(4)— — 
Balance at Dec. 31$7 $5 $5 
Tax Benefits Associated with NOL and Tax Credit Carryforwards Unrecognized tax benefits were reduced by tax benefits associated with NOL and tax credit carryforwards:
(Millions of Dollars)Dec. 31, 2020Dec. 31, 2019
NOL and tax credit carryforwards$(6)$(4)
Interest Payable related to Unrecognized Tax Benefits
Interest payable related to unrecognized tax benefits:
(Millions of Dollars)202020192018
Receivable for interest related to unrecognized tax benefits at Jan. 1$1 $1 $1 
Interest expense related to unrecognized tax benefits(2)— — 
(Payable) receivable for interest related to unrecognized tax benefits at Dec. 31$(1)$1 $1 
NOL and Tax Credit Carryforwards NOL amounts represent the tax loss that is carried forward and tax credits represent the deferred tax asset. NOL and tax credit carryforwards as of Dec. 31 were as follows:
(Millions of Dollars)20202019
Federal tax credit carryforwards$83 $30 
State NOL carryforwards1 1 
Schedule of Effective Income Tax Rate Reconciliation
Total income tax expense from operations differs from the amount computed by applying the statutory federal income tax rate to income before income tax expense.
Effective income tax rate for years ended Dec. 31:
20202019 2018
Federal statutory rate21.0 %21.0 %21.0 %
State income tax on pretax income, net of federal tax effect2.3 2.2 2.3 
Increases (decreases) in tax from:
Wind PTCs(18.3)(7.9)— 
Plant regulatory differences (a)
(6.4)(5.0)(4.8)
Amortization of excess nonplant deferred taxes(0.8)(0.9)(1.2)
Other tax credits, net NOL & tax credit allowances(0.7)(0.6)(0.7)
Adjustments attributable to tax returns(0.6)(0.1)(1.5)
Change in unrecognized tax benefits0.3 0.2 0.1 
Other, net(0.3)0.1 0.3 
Effective income tax rate(3.5)%9.0 %15.5 %
(a)Regulatory differences for income tax primarily relate to the credit of excess deferred taxes to customers through the average rate assumption method. Income tax benefits associated with the credit of excess deferred credits are offset by corresponding revenue reductions.
Schedule of Components of Income Tax Expense (Benefit)
Components of income tax expense for years ended Dec. 31:
(Millions of Dollars)202020192018
Current federal tax (benefit) expense$(31)$(4)$12 
Current state tax (benefit) expense(1)1 3 
Current change in unrecognized tax expense— — 2 
Deferred federal tax expense13 22 20 
Deferred state tax expense8 6 4 
Deferred change in unrecognized tax expense (benefit)1 1 (2)
Total income tax (benefit) expense$(10)$26 $39 
Components of deferred income tax expense as of Dec. 31:
(Millions of Dollars)202020192018
Deferred tax expense excluding items below$53 $53 $44 
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities(31)(24)(22)
Deferred tax expense $22 $29 $22 
Schedule of Deferred Tax Assets and Liabilities
Components of the net deferred tax liability as of Dec. 31:
(Millions of Dollars)2020
2019 (a)
Deferred tax liabilities:
Differences between book and tax bases of property$838 $759 
Operating lease assets109 116 
Regulatory assets59 50 
Pension expense33 33 
Other2 — 
Total deferred tax liabilities$1,041 $958 
Deferred tax assets:
Operating lease liabilities$109 $116 
Regulatory liabilities104 111 
Tax credit carryforward83 30 
Deferred fuel costs9 18 
Other employee benefits7 6 
Other4 5 
Total deferred tax assets316 286 
Net deferred tax liability$725 $672 
(a) Prior periods have been reclassified to conform to current year presentation.