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Income Taxes Income Taxes (Tables) | 6 Months Ended
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Jun. 30, 2019
Income Tax Disclosure [Abstract] |
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] | 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,
| 2019 |  | 2018 |  | 2019 |  | 2018
Federal income taxes at statutory rates | $ | 4,874 |  | 21.0 | % |  | $ | 6,479 |  | 21.0 | % |  | $ | 35,513 |  | 21.0 | % |  | $ | 20,269 |  | 21.0 | %
Increase (decrease) in tax resulting from: |  |  |  |  |  |  |  |  |  |  |
Tax effect of regulatory treatment of utility plant differences | (2,139 | ) | (9.2 | ) |  | (1,981 | ) | (6.4 | ) |  | (4,219 | ) | (2.5 | ) |  | (2,999 | ) | (3.1 | )
State income tax expense | (8 | ) | — |  |  | 60 |  | 0.2 |  |  | 1,651 |  | 1.0 |  |  | 1,024 |  | 1.1 |
Acquisition costs | 112 |  | 0.5 |  |  | 73 |  | 0.2 |  |  | (1,712 | ) | (1.0 | ) |  | 119 |  | 0.1 |
Non-plant excess deferred turnaround (1) | (5,091 | ) | (21.9 | ) |  | (11 | ) | — |  |  | (5,601 | ) | (3.3 | ) |  | (11 | ) | — |
Tax loss on sale of METALfx | (1,259 | ) | (5.4 | ) |  | — |  | — |  |  | (1,259 | ) | (0.8 | ) |  | — |  | — |
Valuation allowance | 1,245 |  | 5.3 |  |  | — |  | — |  |  | 1,245 |  | 0.7 |  |  | — |  | — |
Settlement of equity awards | — |  | — |  |  | — |  | — |  |  | 612 |  | 0.4 |  |  | (990 | ) | (1.0 | )
Other | 525 |  | 2.2 |  |  | 589 |  | 1.9 |  |  | 2,046 |  | 1.2 |  |  | (1,493 | ) | (1.6 | )
Total income tax expense (benefit) | $ | (1,741 | ) | (7.5 | )% |  | $ | 5,209 |  | 16.9 | % |  | $ | 28,276 |  | 16.7 | % |  | $ | 15,919 |  | 16.5 | %
(1) In March 2019, the IPUC approved an all-party settlement agreement related to electric tax benefits that were set aside for Colstrip in the 2017 general rate case order. In the approved settlement agreement, the parties agreed to utilize approximately $6.4 million ($5.1 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, 2027. In the second quarter 2019, the Company recorded a one-time charge to depreciation expense with an offsetting amount included in 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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