Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Feb. 21, 2019
Entity Information [Line Items]
Entity Registrant Name	Northwest Pipeline LLC
Entity Central Index Key	0000110019
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	--12-31
Entity Shell Company	false
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		0
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float		$ 0