DEBT - Trust Preferred Securities (Details) $ in Millions	Sep. 30, 2018 USD ($) shares	Dec. 31, 2017 USD ($)
Trust Preferred Securities
Trust preferred securities	$ 1,700	$ 1,700
Liquidation value	2,570
Junior subordinated debentures owned by the Trust, amount	$ 2,576
Citigroup Capital III
Trust Preferred Securities
Securities issued (in shares) | shares	194,053
Liquidation value	$ 194
Coupon rate	7.625%
Common shares issued to parent (in shares) | shares	6,003
Junior subordinated debentures owned by the Trust, amount	$ 200
Citigroup Capital XIII
Trust Preferred Securities
Securities issued (in shares) | shares	89,840,000
Liquidation value	$ 2,246
Common shares issued to parent (in shares) | shares	1,000
Junior subordinated debentures owned by the Trust, amount	$ 2,246
Citigroup Capital XVIII
Trust Preferred Securities
Securities issued (in shares) | shares	99,901
Liquidation value	$ 130
Common shares issued to parent (in shares) | shares	50
Junior subordinated debentures owned by the Trust, amount	$ 130
LIBOR | Citigroup Capital XIII
Trust Preferred Securities
Coupon rate - basis spread on variable rate	0.0637
LIBOR | Citigroup Capital XVIII
Trust Preferred Securities
Coupon rate - basis spread on variable rate	0.008875