RETIREMENT BENEFITS - Post Employment and Defined Contribution Plans (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Defined Contribution Plans
Maximum percentage contribution by employer of employees eligible pay			6.00%	6.00%
Maximum compensation to be eligible for fixed contribution from employer			$ 100,000
Percentage of fixed contribution by employer, for eligible employees whose compensation is $100,000 or less			2.00%
Pretax expense associated with Citigroup 401(k) plan	$ 94,000,000	$ 93,000,000	$ 295,000,000	$ 298,000,000
Postemployment Plans
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Benefits earned during the period	0	0	0	0
Interest cost on benefit obligation	1,000,000	2,000,000	3,000,000	4,000,000
Amortization of unrecognized
Prior service benefit	(8,000,000)	(8,000,000)	(23,000,000)	(23,000,000)
Net actuarial loss	3,000,000	3,000,000	9,000,000	10,000,000
Total service-related benefit	(4,000,000)	(3,000,000)	(11,000,000)	(9,000,000)
Non-service-related (benefit) expense	9,000,000	4,000,000	15,000,000	21,000,000
Total net expense	$ 5,000,000	$ 1,000,000	$ 4,000,000	$ 12,000,000