RETIREMENT BENEFITS - Defined Contribution Plans and Postemployment Plans (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Defined Contribution Plans
Maximum percentage contribution by employer of employees eligible pay	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%
U.S. Postretirement Benefit Plans
Defined Benefit Plan, Net Periodic Benefit Cost
Interest cost on benefit obligation	$ 1,000,000	$ 1,000,000
Amortization of unrecognized
Prior service benefit	(8,000,000)	(7,000,000)
Net actuarial loss	1,000,000	3,000,000
Total service-related benefit	(6,000,000)	(3,000,000)
Non-service-related (benefit) expense	8,000,000	9,000,000
Total net expense	2,000,000	6,000,000
U.S. Postretirement Benefit Plans
Defined Contribution Plans
Pretax expense associated with Citigroup 401(k) plan	96,000,000	101,000,000
Non-U.S. Postretirement Benefit Plans
Defined Contribution Plans
Pretax expense associated with Citigroup 401(k) plan	$ 68,000,000	$ 74,000,000