Environmental Matters - Regulatory Deferred Assets - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Site Contingency [Line Items]
Regulatory Assets, Current	$ 29,740	$ 41,929	$ 47,996
Regulatory Assets, Noncurrent	$ 324,176	$ 343,146	$ 313,890
Remediation Non-recovery Percentage	3.30%	3.30%	3.30%
Annual tariff rider collection	$ 5,000
Annual insurance proceeds to apply against remediation costs	5,000
Total annual remediation expense and interest	$ 10,000
Oregon [Member]
Site Contingency [Line Items]
Remediation Recovery Percentage	96.70%	96.70%	96.70%
Front Street [Member] | Environmental costs [Member]
Site Contingency [Line Items]
Accrued site liabilities	$ 400	$ 400	$ 400
Northwest Natural Gas Company [Member]
Site Contingency [Line Items]
Regulatory Assets, Current	29,740	41,929	47,996
Regulatory Assets, Noncurrent	324,136	343,146	313,890
Northwest Natural Gas Company [Member] | Environmental costs [Member]
Site Contingency [Line Items]
Deferred costs and interest	35,919	36,673	36,392
Accrued site liabilities	122,226	135,662	118,699
Insurance proceeds and interest	(69,138)	(79,949)	(80,741)
Regulatory Assets	89,007	92,386	74,350
Regulatory Assets, Current	4,440	4,762	5,100
Regulatory Assets, Noncurrent	$ 84,567	$ 87,624	$ 69,250