Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2023	Aug. 02, 2023
Document and Entity Information [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document period end date	Jun. 30, 2023
Document Transition Report	false
Entity File Number	1-7414
Entity registrant name	Northwest Pipeline LLC
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-1157701
Entity Address, Address Line One	One Williams Center
Entity Address, City or Town	Tulsa
Entity Address, State or Province	OK
Entity Address, Postal Zip Code	74172-0172
City Area Code	800
Local Phone Number	945-5426
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity filer category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity central index key	0000110019
Amendment flag	false
Document fiscal year focus	2023
Document fiscal period focus	Q2
Current fiscal year end date	--12-31
Entity common stock, shares outstanding		0