Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	May 02, 2019
Entity Information [Line Items]
Entity registrant name	Northwest Pipeline LLC
Entity central index key	0000110019
Document type	10-Q
Document period end date	Mar. 31, 2019
Amendment flag	false
Document fiscal year focus	2019
Document fiscal period focus	Q1
Current fiscal year end date	--12-31
Entity filer category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity common stock, shares outstanding		0