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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$7 $7 
Unrecognized tax benefit — Temporary tax positions2 5 
Total unrecognized tax benefit$9 $12 
Changes in unrecognized tax benefits:
(Millions of Dollars)202020192018
Balance at Jan. 1$12 $10 $10 
Additions based on tax positions related to the current year2 1 1 
Additions for tax positions of prior years6 1 — 
Reductions for tax positions of prior years(11)— — 
Settlements with taxing authorities— — (1)
Balance at Dec. 31$9 $12 $10 
Tax Benefits Associated with NOL and Tax Credit Carryforwards
Unrecognized tax benefits were reduced by tax benefits associated with NOL and tax credit carryforwards:
(Millions of Dollars)Dec. 31, 2020Dec. 31, 2019
NOL and tax credit carryforwards$(8)$(8)
Interest Payable related to Unrecognized Tax Benefits
Interest payable related to unrecognized tax benefits:
(Millions of Dollars)202020192018
Payable for interest related to unrecognized tax benefits at Jan. 1$(1)$(1)$— 
Interest income (expense) related to unrecognized tax benefits1 — (1)
Payable for interest related to unrecognized tax benefits at Dec. 31$— $(1)$(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$143 $83 
State NOL carryforwards190 388 
State tax credit carryforwards, net of federal detriment (a)
18 18 
Valuation allowances for state credit carryforwards, net of federal benefit (b)
(8)(8)
(a)State tax credit carryforwards are net of federal detriment of $5 million as of both Dec. 31, 2020 and 2019.
(b)Valuation allowances for state tax credit carryforwards were net of federal benefit of $2 million as of both Dec. 31, 2020 and 2019.
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:
2020
2019
2018
Federal statutory rate21.0 %21.0 %21.0 %
State income tax on pretax income, net of federal tax effect3.6 3.6 3.7 
Increases (decreases) in tax from:
Wind PTCs(10.3)(7.5)(0.6)
Plant regulatory differences (a)
(5.0)(3.3)(4.5)
Other tax credits, net NOL & tax credit allowances(1.1)(1.3)(0.6)
Amortization of excess nonplant deferred taxes(0.2)(0.2)(1.4)
Change in unrecognized tax benefits(0.2)0.3 0.1 
Other, net(0.7)(0.4)(0.7)
Effective income tax rate7.1 %12.2 %17.0 %
.
Schedule of Components of Income Tax Expense (Benefit)
Components of income tax expense for the years ended Dec. 31:
(Millions of Dollars)202020192018
Current federal tax expense (benefit)$44 $(9)$79 
Current state tax expense (benefit)4 (5)14 
Current change in unrecognized tax benefit(3)(1)(1)
Deferred federal tax (benefit) expense(26)61 5 
Deferred state tax expense26 33 17 
Deferred change in unrecognized tax expense2 3 2 
Deferred ITCs(2)(2)(3)
Total income tax expense$45 $80 $113 
Components of deferred income tax expense as of Dec. 31:
(Millions of Dollars)202020192018
Deferred tax expense excluding items below$46 $132 $75 
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities(43)(35)(51)
Tax expense allocated to other comprehensive income, adoption of ASC Topic 326, adoption of ASU No. 2018-02, and other(1)— — 
Deferred tax expense$2 $97 $24 
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$2,132 $2,039 
Regulatory assets257 253 
Operating lease assets129 148 
Pension expense and other employee benefits19 22 
Other6 7 
Total deferred tax liabilities$2,543 $2,469 
Deferred tax assets:
Regulatory liabilities$319 $327 
Tax credit carryforward161 101 
Operating lease liabilities129 148 
Bad debts8 5 
NOL carryforward7 14 
Deferred ITCs5 6 
Tax credit valuation allowances(8)(8)
Other25 25 
Total deferred tax assets$646 $618 
Net deferred tax liability$1,897 $1,851 
(a) Prior periods have been reclassified to conform to current year presentation.