Income Tax - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income taxes at statutory rates (federal and state)	$ (5,048)	$ (4,799)	$ 6,628	$ 6,703
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount	(1,452)	(1,708)	1,892	1,797
Differences required to be flowed-through by regulatory commissions	1,051	2,090	(1,301)	(3,207)
Other, net	86	74	(127)	(336)
Total provision for income taxes on continuing operations	$ (5,363)	$ (4,343)	$ 7,092	$ 4,957
Effective tax rate for continuing operations	22.30%	19.00%	22.50%	15.50%
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%		35.00%
Northwest Natural Gas Company [Member]
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income taxes at statutory rates (federal and state)	$ (5,518)	$ (4,536)	$ 6,550	$ 7,388
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount	(1,595)	(1,641)	1,861	1,975
Differences required to be flowed-through by regulatory commissions	1,051	2,090	(1,301)	(3,207)
Other, net	5	74	(168)	(336)
Total provision for income taxes on continuing operations	$ (6,057)	$ (4,013)	$ 6,942	$ 5,820
Effective tax rate for continuing operations	23.10%	18.60%	22.30%	16.50%