Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Oct. 27, 2017
Document and Entity Information [Abstract]
Document Type	10-Q
Document Period End Date	Sep. 30, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Central Index Key	0000073020
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Registrant Name	NORTHWEST NATURAL GAS CO
Entity Voluntary Filers	No
Entity Well Known Season Issuer	Yes
Entity Common Stock, Shares Outstanding (in shares)		28,713,052
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q3