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Income Taxes Income Taxes (Tables) | 9 Months Ended
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Sep. 30, 2018
Income Tax Disclosure [Abstract] |
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] | The following table summarizes the significant differences in income tax expense based on the differences between our effective tax rate and the federal statutory rates (21 percent in 2018 and 35 percent in 2017) for three and nine months ended September 30 (dollars in thousands):
| Three months ended September 30, |  | Nine months ended September 30,
| 2018 |  | 2017 |  | 2018 |  | 2017
Federal income taxes at statutory rates | $ | 2,452 |  | 21.0 | % |  | $ | 3,364 |  | 35.0 | % |  | $ | 22,721 |  | 21.0 | % |  | $ | 48,953 |  | 35.0 | %
Increase (decrease) in tax resulting from: |  |  |  |  |  |  |  |  |  |  |
Tax effect of regulatory treatment of utility plant differences | (1,521 | ) | (13.0 | ) |  | 742 |  | 7.7 |  |  | (4,519 | ) | (4.2 | ) |  | 2,244 |  | 1.6 |
State income tax expense | (319 | ) | (2.7 | ) |  | (301 | ) | (3.1 | ) |  | 694 |  | 0.6 |  |  | 1,136 |  | 0.8 |
Acquisition costs | 122 |  | 1.0 |  |  | 1,997 |  | 20.8 |  |  | 241 |  | 0.2 |  |  | 2,317 |  | 1.7 |
Settlement of equity awards | — |  | — |  |  | — |  | — |  |  | (990 | ) | (0.9 | ) |  | (1,439 | ) | (1.0 | )
Other | 814 |  | 7.0 |  |  | (649 | ) | (6.8 | ) |  | (680 | ) | (0.6 | ) |  | (1,663 | ) | (1.2 | )
Total income tax expense | $ | 1,548 |  | 13.3 | % |  | $ | 5,153 |  | 53.6 | % |  | $ | 17,467 |  | 16.1 | % |  | $ | 51,548 |  | 36.9 | %

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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://www.xbrl.org/2003/role/presentationRef -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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