Document And Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Feb. 26, 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-K
Document Period End Date	Dec. 31, 2014
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding	480,094,891	481,501,265
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 10,478,726,281