Transactions with Major Customers and Affiliates (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) employee	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017
Related Party Transaction [Line Items]
Advances to affiliate			$ 45,137		$ 137,666
Related party transaction, rate		1.16%
Entity number of employees		0				0
Related party transaction, expenses from transactions with related party		$ 96,500	92,800	$ 100,400
Severance Costs			2,400
Cash distributions to parent		186,000	174,000	168,000
Subsequent Event [Member]
Related Party Transaction [Line Items]
Cash distributions to parent	$ 34,000
Puget Sound Energy, Inc. [Member]
Revenue, Major Customer [Line Items]
Operating revenues		120,226	120,351	118,384
Northwest Natural Gas Company [Member]
Revenue, Major Customer [Line Items]
Operating revenues		51,743	49,895	50,857
Cascade Natural Gas Corporation [Member]
Revenue, Major Customer [Line Items]
Operating revenues		$ 48,071	$ 47,951	$ 48,363