RETIREMENT BENEFITS - Post Employment and Defined Contribution Plans (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 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	$ 99,000,000	$ 101,000,000	$ 200,000,000	$ 204,000,000
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	1,000,000	2,000,000	2,000,000
Amortization of unrecognized
Prior service cost (benefit)	(8,000,000)	(8,000,000)	(15,000,000)	(15,000,000)
Net actuarial loss	3,000,000	3,000,000	6,000,000	7,000,000
Total service related expense	(4,000,000)	(4,000,000)	(7,000,000)	(6,000,000)
Non-service related expense (benefit)	(3,000,000)	8,000,000	6,000,000	17,000,000
Total net (benefit) expense	$ (7,000,000)	$ 4,000,000	$ (1,000,000)	$ 11,000,000