Discontinued Operations Agreement (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2019
Other Ownership Interests [Line Items]
Disposal Group, Including Discontinued Operation, Description and Timing of Disposal	The Agreement provides for an initial cash purchase price of $25.0 million (subject to a working capital adjustment), plus potential additional payments to NWN Gas Storage of up to $26.5 million in the aggregate if Gill Ranch achieves certain economic performance levels for the first three full gas storage years (April 1 of one year through March 31 of the following year) occurring after the closing and the remaining portion of the gas storage year during which the closing occurs. We expect the transaction to close within 12 months of signing and in 2019. The closing of the transaction is subject to approval by the California Public Utilities Commission (CPUC) and other customary closing conditions.
Gill Ranch [Member]
Other Ownership Interests [Line Items]
Percent Ownership In Joint Project	75.00%
Pacific Gas and Electric Company [Member]
Other Ownership Interests [Line Items]
Percent Ownership In Joint Project	25.00%
Minimum | Scenario, Forecast [Member]
Other Ownership Interests [Line Items]
Proceeds from Divestiture of Businesses		$ 25.0
Maximum | Scenario, Forecast [Member]
Other Ownership Interests [Line Items]
Proceeds from Divestiture of Businesses		$ 26.5