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INCOME TAXES
3 Months Ended
Mar. 31, 2021
Income Tax Disclosure [Abstract]  
INCOME TAXES INCOME TAXES
The provision for income taxes differs from the amount of income tax determined by applying the applicable United States statutory federal income tax rate to income before income taxes as a result of the following:
Three Months Ended March 31, 2021Three Months Ended March 31, 2020
(in millions)AmountEffective Tax RateAmountEffective Tax Rate
Statutory federal income tax$122.8 21.0 %$113.9 21.0 %
State income taxes net of federal tax benefit36.9 6.3 %34.0 6.3 %
PTCs(34.0)(5.8)%(18.4)(3.4)%
Federal excess deferred tax amortization – Wisconsin unprotected(30.3)(5.2)%(22.1)(4.1)%
Federal excess deferred tax amortization(14.6)(2.5)%(13.0)(2.4)%
Uncertain tax positions(8.2)(1.4)%— — %
Other2.3 0.4 %(4.4)(0.8)%
Total income tax expense$74.9 12.8 %$90.0 16.6 %

The effective tax rates of 12.8% and 16.6% for the three months ended March 31, 2021 and 2020, respectively, differ from the United States statutory federal income tax rate of 21%, primarily due to PTCs generated from ownership interests in wind generation facilities in our non-utility energy infrastructure segment and the recognition of certain unprotected deferred tax benefits created as a result of the Tax Legislation. In accordance with the rate order received from the PSCW in December 2019, our Wisconsin utilities are amortizing the unprotected deferred tax benefits over periods ranging from two years to four years, to reduce near-term rate impacts to their customers. In addition, the impact of the protected benefits associated with the Tax Legislation, as discussed in more detail below, drove a decrease in the effective tax rate, which was partially offset by state income taxes.

The Tax Legislation required our regulated utilities to remeasure their deferred income taxes and we began to amortize the resulting excess protected deferred income taxes beginning in 2018 in accordance with normalization requirements (see federal excess deferred tax amortization line above).

See Note 22, Regulatory Environment, for more information on unprotected tax credits.