Document and Entity Information - shares	12 Months Ended
	Dec. 31, 2020	Feb. 24, 2021
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
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	295 Chipeta Way
Entity Address, City or Town	Salt Lake City
Entity Address, State or Province	UT
Entity Address, Postal Zip Code	84108
City Area Code	801
Local Phone Number	583-8800
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
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	2020
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	--12-31
Entity Common Stock, Shares Outstanding		0