Document And Entity Information - USD ($)	9 Months Ended
	Sep. 30, 2015	Oct. 28, 2015	Jun. 30, 2014
Document And Entity Information [Abstract]
Entity Registrant Name	Ally Financial Inc.
Entity Central Index Key	0000040729
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-Q
Document Period End Date	Sep. 30, 2015
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q3
Amendment Flag	false
Entity Common Stock, Shares Outstanding		481,752,179
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 10,478,726,281