Environmental Matters - Regulatory Deferred Assets (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Site Contingency [Line Items]
Regulatory Assets, Current	$ 37,815	$ 41,929	$ 46,317
Regulatory Assets, Noncurrent	$ 328,024	$ 343,146	$ 327,194
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
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	37,815	41,929	46,317
Regulatory Assets, Noncurrent	328,024	343,146	327,194
Northwest Natural Gas Company [Member] | Environmental costs [Member]
Site Contingency [Line Items]
Deferred costs and interest	37,586	36,673	36,874
Accrued site liabilities	129,977	135,662	124,133
Insurance proceeds and interest	(80,613)	(79,949)	(89,305)
Regulatory Assets	86,950	92,386	71,702
Regulatory Assets, Current	4,459	4,762	5,090
Regulatory Assets, Noncurrent	$ 82,491	$ 87,624	$ 66,612