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Income Taxes (Tables) | 12 Months Ended
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Dec. 31, 2020
Income Tax Disclosure [Abstract] |
Summary of Effective Income Tax Rates | The components of income before income taxes for each of the three years ended December 31, 2020 and income tax expense (benefit) attributable thereto were as follows:
| Years Ended December 31,
(Millions of dollars) | 2020 |  | 2019 |  | 2018
Income (loss) before income taxes | $ | 509.1 |  |  | $ | 202.4 |  |  | $ | 273.9 |
Income tax expense (benefit) |  |  |  |  |
Federal - Current | $ | 96.0 |  |  | 15.6 |  |  | 18.4 |
Federal - Deferred | 4.7 |  |  | 21.7 |  |  | 31.0 |
State - Current and deferred | 22.3 |  |  | 10.3 |  |  | 10.9 |
Total | $ | 123.0 |  |  | $ | 47.6 |  |  | $ | 60.3 |
Schedule of Reconciliation of Income Taxes to Statutory Rate | The following table reconciles income taxes based on the U.S. statutory tax rate to the Company’s income tax expense (benefit).
| Years Ended December 31,
(Millions of dollars) | 2020 |  | 2019 |  | 2018
Income tax expense based on the U.S. statutory tax rate | $ | 106.9 |  |  | $ | 42.5 |  |  | $ | 57.5 |
State income taxes, net of federal benefit | 17.5 |  |  | 8.6 |  |  | 8.3 |
Federal credits | (1.9) |  |  | (2.3) |  |  | (2.0) |
Other, net | 0.5 |  |  | (1.2) |  |  | (3.5) |
Total | $ | 123.0 |  |  | $ | 47.6 |  |  | $ | 60.3 |
Summary of Deferred Tax Assets and Deferred Tax Liabilities | An analysis of the Company’s deferred tax assets and deferred tax liabilities at December 31, 2020 and 2019 showing the tax effects of significant temporary differences is as follows:
| December 31,
(Millions of dollars) | 2020 |  | 2019
Deferred tax assets |  |  |
Property costs and asset retirement obligations | $ | 4.5 |  |  | $ | 3.7 |
Employee benefits | 8.0 |  |  | 6.1 |
Operating leases liability | 31.6 |  |  | 25.0 |
Other deferred tax assets | 7.2 |  |  | 2.1 |
Total gross deferred tax assets | 51.3 |  |  | 36.9 |
Deferred tax liabilities |  |  |
Accumulated depreciation and amortization | (213.2) |  |  | (191.2) |
State deferred taxes | (20.2) |  |  | (27.9) |
Operating leases right of use assets | (31.0) |  |  | (24.8) |
Other deferred tax liabilities | (5.3) |  |  | (9.7) |
Total gross deferred tax liabilities | (269.7) |  |  | (253.6) |
Net deferred tax liabilities | $ | (218.4) |  |  | $ | (216.7) |
Reconciliation of Beginning and Ending Liability for Uncertain Tax Positions | A reconciliation of the beginning and ending amount of the consolidated liability for unrecognized income tax benefits during the year ended December 31, 2020 and 2019 is shown in the following table.
| Year Ended December 31,
(Millions of dollars) | 2020 |  | 2019
Balance at January 1 | $ | 0.6 |  |  | $ | 0.7 |
Additions for tax positions related to prior years | 0.1 |  |  | 0.5 |
Settlements with taxing authorities | (0.3) |  |  | (0.6) |
Balance at December 31 | $ | 0.4 |  |  | $ | 0.6 |

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