Environmental Matters - Regulatory Deferred Assets (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Site Contingency [Line Items]
Regulatory Assets, Current	$ 30,021	$ 46,688	$ 41,929
Regulatory Assets, Noncurrent	$ 324,358	$ 318,340	$ 343,146
Remediation Non-recovery Percentage	3.32%	3.32%	3.32%
Annual tariff rider collection	$ 5,000
Annual insurance proceeds to apply against remediation costs	5,000
Total annual remediation expense and interest	$ 10,000
Environmental Earnings Test Reserve		$ 4,400
Oregon [Member]
Site Contingency [Line Items]
Remediation Recovery Percentage	96.68%	96.68%	96.68%
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	30,021	46,688	41,929
Regulatory Assets, Noncurrent	324,320	318,340	343,146
Northwest Natural Gas Company [Member] | Environmental costs [Member]
Site Contingency [Line Items]
Deferred costs and interest	42,098	43,222	36,673
Accrued site liabilities	124,981	120,226	135,662
Insurance proceeds and interest	(81,146)	(90,119)	(79,949)
Regulatory Assets	85,933	73,329	92,386
Regulatory Assets, Current	4,176	5,089	4,762
Regulatory Assets, Noncurrent	$ 81,757	$ 68,240	$ 87,624