Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2018	Aug. 02, 2018
Entity Information [Line Items]
Entity registrant name	Northwest Pipeline LLC
Entity central index key	0000110019
Document type	10-Q
Document period end date	Jun. 30, 2018
Amendment flag	false
Document fiscal year focus	2018
Document fiscal period focus	Q2
Current fiscal year end date	--12-31
Entity filer category	Non-accelerated Filer
Entity common stock, shares outstanding		0