Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2019	Feb. 21, 2020	Jun. 28, 2019
Entity Information [Line Items]
Document Annual Report	true
Entity Public Float			$ 12.2
Document Type	10-K
Document Period End Date	Dec. 31, 2019
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
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
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		375,074,939
Entity Central Index Key	0000040729
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Amendment Flag	false
Common stock
Entity Information [Line Items]
Security Exchange Name	NYSE
Title of 12(b) Security	Common Stock, par value $0.01 per share
Trading Symbol	ALLY
Trust Preferred Securities Subject to Mandatory Redemption
Entity Information [Line Items]
Security Exchange Name	NYSE
Title of 12(b) Security	8.125% Fixed Rate/Floating Rate Trust Preferred Securities, Series 2 of GMAC Capital Trust I
Trading Symbol	ALLY PRA