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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
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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Amount of public utility's requested rate increase (decrease) with regulatory agency.

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Percentage of public utility's requested rate increase (decrease) with regulatory agency.

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