Transactions with Major Customers and Affiliates (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) employee	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Related Party Transaction [Line Items]
Advances to affiliate		$ 180,400	$ 137,666
Related party transaction, rate		2.24%
Entity number of employees | employee		0
Related party transaction, expenses from transactions with related party		$ 92,700	96,500	$ 92,800
Severance Costs				2,400
Cash distributions to parent		174,000	186,000	174,000
Williams Companies Inc [Member]
Related Party Transaction [Line Items]
Advances to affiliate		180,400	137,666
Subsequent Event [Member]
Related Party Transaction [Line Items]
Cash distributions to parent	$ 21,000
Puget Sound Energy, Inc. [Member]
Revenue, Major Customer [Line Items]
Operating revenues		115,238	120,226	120,351
Northwest Natural Gas Company [Member]
Revenue, Major Customer [Line Items]
Operating revenues		49,126	51,743	49,895
Cascade Natural Gas Corporation [Member]
Revenue, Major Customer [Line Items]
Operating revenues		$ 46,164	$ 48,071	$ 47,951