Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2018	Apr. 30, 2018
Entity Information [Line Items]
Entity Registrant Name	Williams Partners L.P.
Entity Central Index Key	0001483096
Document Type	10-Q
Document Period End Date	Mar. 31, 2018
Amendment Flag	false
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Partnership Units Outstanding		957,529,489
Common Class B [Member]
Entity Information [Line Items]
Entity Partnership Units Outstanding		18,124,096