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Income Taxes - Summary of Significant Factors Impact on Difference Between Effective Tax Rate and Federal Statutory Rate (Parenthetical) (Details) - USD ($) $ in Thousands | 1 Months Ended | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | ---
Mar. 31, 2020 | Mar. 31, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2019
Effective Tax Rate Reconciliation [Line Items] |  |  |  |  |  |  |
Regulatory liabilities |  |  | $ 48,902 |  | $ 48,902 |  | $ 51,715
Non-plant excess deferred turnaround, amount |  |  | 0 | $ (20) | (8,476) | $ (5,621) |
WUTC |  |  |  |  |  |  |
Effective Tax Rate Reconciliation [Line Items] |  |  |  |  |  |  |
Non-plant excess deferred turnaround, amount | $ 8,400 |  |  |  |  |  |
IPUC |  |  |  |  |  |  |
Effective Tax Rate Reconciliation [Line Items] |  |  |  |  |  |  |
Non-plant excess deferred turnaround, amount |  | $ 5,100 |  |  |  |  |
Income Tax Related |  |  |  |  |  |  |
Effective Tax Rate Reconciliation [Line Items] |  |  |  |  |  |  |
Regulatory liabilities |  |  | $ 13,598 |  | $ 13,598 |  | $ 23,803
Income Tax Related | WUTC |  |  |  |  |  |  |
Effective Tax Rate Reconciliation [Line Items] |  |  |  |  |  |  |
Regulatory liabilities | $ 10,900 |  |  |  |  |  |
Income Tax Related | IPUC |  |  |  |  |  |  |
Effective Tax Rate Reconciliation [Line Items] |  |  |  |  |  |  |
Regulatory liabilities |  | $ 6,400 |  |  |  |  |

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