DISCONTINUED OPERATIONS AND SIGNIFICANT DISPOSALS - Discontinued Operations (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 transaction
Results of Discontinued Operations
Total revenues, net of interest expense	$ 0	$ 2	$ 0	$ 75
Income (loss) from discontinued operations	(15)	(25)	(14)	12
Provision for income taxes	(5)	(9)	(5)	13
Income (loss) from discontinued operations, net of taxes	(10)	(16)	(9)	(1)
Brazil Creditcard
Results of Discontinued Operations
Income (loss) from discontinued operations, net of taxes	0	(3)	$ 6	53
Citi Capital Advisors Business
Discontinued operations
Number of transactions | transaction						2
Ownership interest (as a percent)			24.90%
Results of Discontinued Operations
Income (loss) from discontinued operations, net of taxes	0	(3)	$ 1	(5)
Egg Banking PLC
Results of Discontinued Operations
Income (loss) from discontinued operations, net of taxes	$ (10)	$ (10)	$ (16)	$ (29)
German retail banking operations
Discontinued operations
Residual costs associated with banking operations tax effect					$ 20