DISCONTINUED OPERATIONS, SIGNIFICANT DISPOSALS AND OTHER BUSINESS EXITS - Narrative (Details) $ in Millions	3 Months Ended			7 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2024 USD ($) business	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) business	Sep. 30, 2023 USD ($)	Jun. 30, 2024 USD ($)	Dec. 31, 2022 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (loss) from discontinued operations, net of taxes	$ (1)	$ 2			$ (2)	$ 0
Cost incurred to date	$ 1,051				$ 1,051
Personal installment loan
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of loans			$ 5	$ (7)
Severance | Russian Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Settlement of credit card receivable							$ 55
Citibank Korea Inc. | Severance
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cost incurred to date								$ 1,100
Legacy Franchises | Consumer banking businesses
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of businesses with sale agreements | business	9				9
Number of businesses sold | business	5				5