Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	May 04, 2020
Document and Entity Information [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document period end date	Mar. 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 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	Q1
Current fiscal year end date	--12-31
Entity common stock, shares outstanding		0