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Income Taxes (Tables) | 6 Months Ended
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Jun. 30, 2021
Income Tax Disclosure [Abstract] |
Summary of Significant Factors Impact on Difference Between Effective Tax Rate and Federal Statutory Rate | The following table summarizes the significant factors impacting the difference between our effective tax rate and the federal statutory rate for the three and six months ended June 30 (dollars in thousands):
|  | Three months ended June 30, |  |  | Six months ended June 30, |
|  | 2021 |  |  | 2020 |  |  | 2021 |  |  | 2020 |
Federal income taxes at statutory rates |  | $ | 3,459 |  |  |  | 21.0 | % |  | $ | 2,002 |  |  |  | 21.0 | % |  | $ | 20,297 |  |  |  | 21.0 | % |  | $ | 13,963 |  |  |  | 21.0 | %
Increase (decrease) in tax resulting from: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Tax effect of regulatory treatment of utility plant differences |  |  | (3,204 | ) |  |  | (19.5 | ) |  |  | (2,415 | ) |  |  | (25.3 | ) |  |  | (6,444 | ) |  |  | (6.7 | ) |  |  | (4,817 | ) |  |  | (7.2 | )
State income tax expense |  |  | 280 |  |  |  | 1.7 |  |  |  | (325 | ) |  |  | (3.4 | ) |  |  | 832 |  |  |  | 0.9 |  |  |  | 902 |  |  |  | 1.4 |
Non-plant excess deferred turnaround (1) |  |  | — |  |  |  | — |  |  |  | (8,390 | ) |  |  | (88.0 | ) |  |  | — |  |  |  | — |  |  |  | (8,476 | ) |  |  | (12.7 | )
Settlement of equity awards |  |  | (21 | ) |  |  | (0.1 | ) |  |  | — |  |  |  | — |  |  |  | 909 |  |  |  | 1.0 |  |  |  | 165 |  |  |  | 0.1 |
Other |  |  | 1,884 |  |  |  | 11.5 |  |  |  | 1,209 |  |  |  | 12.7 |  |  |  | (1,032 | ) |  |  | (1.1 | ) |  |  | (1,124 | ) |  |  | (1.7 | )
Total income tax expense |  | $ | 2,398 |  |  |  | 14.6 | % |  | $ | (7,919 | ) |  |  | (83.0 | )% |  | $ | 14,562 |  |  |  | 15.1 | % |  | $ | 613 |  |  |  | 0.9 | %
(1) In March 2020, the WUTC approved an all-party settlement agreement related to electric tax benefits that were set aside for Colstrip in the 2020 general rate case order. In the approved settlement agreement, the parties agreed to utilize $10.9 million ($8.4 million when tax-effected) of the electric tax benefits to offset costs associated with accelerating the depreciation of Colstrip Units 3 & 4, to reflect a remaining useful life of those units through December 31, 2025. In the second quarter 2020, the Company recorded a one-time increase to depreciation expense with an offsetting decrease to income tax expense.

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- Definition Tabular disclosure of the reconciliation using percentage or dollar amounts of the reported amount of income tax expense attributable to continuing operations for the year to the amount of income tax expense that would result from applying domestic federal statutory tax rates to pretax income from continuing operations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319 + Details
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