Document and Entity Information (USD $)	12 Months Ended
	Sep. 30, 2012	Nov. 15, 2012	Mar. 30, 2012
Entity Information [Line Items]
Document Type	10-K
Amendment Flag	false
Entity Common Stock, Shares Outstanding		0
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Document Period End Date	Sep. 30, 2012
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	FY
Trading Symbol	NRGM
Entity Registrant Name	INERGY MIDSTREAM, L.P.
Entity Central Index Key	0001304464
Current Fiscal Year End Date	--09-30
Entity Filer Category	Non-accelerated Filer
Entity Current Reporting Status	Yes
Entity Public Float			$ 382,100,000