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Washington Utilities and Transportation Commission (WUTC) (Details 5) - WUTC - USD ($) $ in Millions | Oct. 11, 2018 | Jul. 20, 2018 | Jun. 01, 2018 | Mar. 23, 2018 | Aug. 31, 2017
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Natural gas rate proceeding |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Public utilities, requested rate increase (decrease), amount |  |  |  |  | $ 5.9
Public utilities, requested rate increase (decrease), percentage |  |  |  |  | 2.70%
Natural gas rate proceeding | Tax Cuts and Jobs Act |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Public utilities, requested rate increase (decrease), amended, amount |  |  |  | $ (1.7) |
Public Utilities, Approved Rate Increase (Decrease), Amount |  | $ (2.9) |  |  |
Public utilities, approved rate increase (decrease), percentage |  | (1.40%) |  |  |
Public Utilities, Approved Rate Increase (Decrease) For Excess Deferred Income Taxes, Amount |  | $ (2.5) |  |  |
Public Utilities, Approved Rate Increase (Decrease) For Tax Benefits, Amount |  | $ (1.6) |  |  |
Pipeline replacement cost recovery |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Public utilities, requested rate increase (decrease), amount |  |  | $ 2.3 |  |
Public utilities, requested rate increase (decrease), percentage |  |  | 1.10% |  |
Pipeline replacement cost recovery | Subsequent Event |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Public utilities, requested rate increase (decrease), amount | $ 2.1 |  |  |  |
Public utilities, requested rate increase (decrease), percentage | 1.00% |  |  |  |

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