Environmental Matters - Regulatory Deferred Assets (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Site Contingency [Line Items]
Regulatory Assets, Current	$ 46,317	$ 41,930	$ 45,900
Regulatory Assets, Noncurrent	$ 327,194	$ 371,786	$ 343,037
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	46,317	41,930	45,900
Regulatory Assets, Noncurrent	327,194	371,786	343,037
Northwest Natural Gas Company [Member] | Environmental costs [Member]
Site Contingency [Line Items]
Deferred costs and interest	36,874	41,883	44,686
Accrued site liabilities	124,133	128,369	122,962
Insurance proceeds and interest	(89,305)	(88,502)	(95,100)
Regulatory Assets	71,702	81,750	72,548
Regulatory Assets, Current	5,090	5,601	5,818
Regulatory Assets, Noncurrent	$ 66,612	$ 76,149	$ 66,730