Document and Entity Information (USD $)	9 Months Ended
	Sep. 30, 2011	Apr. 30, 2012	Mar. 31, 2012
Document and Entity Information [Abstract]
Document Type	10-Q
Document period end date	Mar. 31, 2012
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		26,704,134
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	FY
Entity public float			$ 1,177,593,524.1