Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2015	Oct. 29, 2015
Entity Information [Line Items]
Entity registrant name	Northwest Pipeline LLC
Entity central index key	0000110019
Document type	10-Q
Document period end date	Sep. 30, 2015
Amendment flag	false
Document fiscal year focus	2015
Document fiscal period focus	Q3
Current fiscal year end date	--12-31
Entity filer category	Non-accelerated Filer
Entity common stock, shares outstanding		0