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Washington Utilities and Transportation Commission (WUTC) (Details 5) - WUTC - Natural gas rate proceeding - USD ($) $ in Millions | Mar. 23, 2018 | Aug. 31, 2017
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Public Utilities, General Disclosures [Line Items] |  |
Public utilities, requested rate increase (decrease), amount |  | $ 5.9
Public utilities, requested rate increase (decrease), percentage |  | 2.70%
Tax Cuts and Jobs Act |  |
Public Utilities, General Disclosures [Line Items] |  |
Public utilities, requested rate increase (decrease), amended, amount | $ (1.7) |

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