Cover Page - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2021	Feb. 23, 2022	Jun. 30, 2021
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	1-3754
Entity Registrant Name	ALLY FINANCIAL INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	38-0572512
Entity Address, Address Description	Ally Detroit Center
Entity Address, Address Line One	500 Woodward Ave.
Entity Address, Address Line Two	Floor 10
Entity Address, City or Town	Detroit
Entity Address, State or Province	MI
Entity Address, Postal Zip Code	48226
City Area Code	866
Local Phone Number	710-4623
Title of 12(b) Security	Common Stock, par value $0.01 per share
Trading Symbol	ALLY
Security Exchange Name	NYSE
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity public float			$ 18.1
Entity Common Stock, Shares Outstanding		333,195,505
Documents Incorporated by Reference	Documents incorporated by reference: portions of the Registrant’s Proxy Statement for the annual meeting of stockholders to be held on May 3, 2022, are incorporated by reference in this Form 10-K in response to Items 10, 11, 12, 13, and 14 of Part III.
Entity Central Index Key	0000040729
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Amendment Flag	false