Document and Entity Information	3 Months Ended
	Mar. 31, 2013	Apr. 26, 2013
Document and Entity Information [Abstract]
Document Type	10-Q
Document Period End Date	Mar. 31, 2013
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 Seasoned Issuer	Yes
Entity Common Stock Shares Outstanding		26,948,572
Document Fiscal Year Focus	2013
Document Fiscal Period Focus	Q1