Environmental Matters - Regulatory Deferred Assets (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Site Contingency [Line Items]
Regulatory Assets, Current	$ 47,996	$ 41,930	$ 41,241
Regulatory Assets, Noncurrent	$ 313,890	$ 371,786	$ 333,917
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	47,996	41,930	41,241
Regulatory Assets, Noncurrent	313,890	371,786	333,917
Northwest Natural Gas Company [Member] | Environmental costs [Member]
Site Contingency [Line Items]
Deferred costs and interest	36,392	41,883	40,578
Accrued site liabilities	118,699	128,369	116,150
Insurance proceeds and interest	(80,741)	(88,502)	(87,631)
Regulatory Assets	74,350	81,750	69,097
Regulatory Assets, Current	5,100	5,601	5,633
Regulatory Assets, Noncurrent	$ 69,250	$ 76,149	$ 63,464