Washington Utilities and Transportation Commission (WUTC) (Details 5) - WUTC - Natural gas rate proceeding - USD ($) $ in Millions	Mar. 23, 2018	Aug. 31, 2017
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)