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Washington Utilities and Transportation Commission (WUTC) (Details 3) - WUTC [Member] - USD ($) $ in Millions | Oct. 26, 2017 | Aug. 31, 2017 | May 31, 2017
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Pipeline replacement cost recovery [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Public utilities, requested rate increase (decrease), amount |  |  | $ 1.6
Public utilities, requested rate increase (decrease), percentage |  |  | 0.75%
Natural gas rate proceeding [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Public utilities, requested rate increase (decrease), amount |  | $ 5.9 |
Public utilities, requested rate increase (decrease), percentage |  | 2.70% |
Subsequent event [Member] | Pipeline replacement cost recovery [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Public utilities, approved rate increase (decrease), amount | $ 1.3 |  |
Public utilities, approved rate increase (decrease), percentage | 0.61% |  |

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