RETIREMENT BENEFITS - Defined Contribution Plans and Postemployment Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
U.S. Postretirement Benefit Plans
Defined Benefit Plan, Net Periodic Benefit Cost
Interest cost on benefit obligation	$ 0	$ 1	$ 2	$ 3
Amortization of unrecognized
Prior service benefit	(7)	(8)	(23)	(23)
Net actuarial loss	1	3	3	9
Total service-related benefit	(6)	(4)	(18)	(11)
Non-service-related expense (benefit)	10	9	23	15
Total net (benefit) expense	4	5	5	4
U.S. Postretirement Benefit Plans
Defined Contribution Plans
Pretax expense associated with Citigroup 401(k) plan	88	94	281	295
Non-U.S. Postretirement Benefit Plans
Defined Contribution Plans
Pretax expense associated with Citigroup 401(k) plan	$ 67	$ 67	$ 207	$ 212