BUSINESS SEGMENTS (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($) country	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) country		Dec. 31, 2016 USD ($)		Dec. 31, 2015 USD ($)
Segment reporting information
Revenues, net of interest expense	$ 17,255	$ 18,173	$ 17,901	$ 18,120	$ 17,012	$ 17,760	$ 17,548	$ 17,555	$ 71,449	[1]	$ 69,875	[1]	$ 76,354	[1]
Provision (benefits) for income taxes	23,864	1,866	1,795	1,863	1,509	1,733	1,723	1,479	29,388		6,444		7,440
Income (loss) from continuing operations	(18,765)	4,137	3,883	4,118	3,591	3,887	4,047	3,508	(6,627)		15,033		17,386
Identifiable assets	1,842,465				1,792,077				1,842,465		1,792,077
Provisions for credit losses and for benefits and claims	2,073	$ 1,999	$ 1,717	$ 1,662	1,792	$ 1,736	$ 1,409	$ 2,045	7,451		6,982		7,913
Operating Segments | Citicorp | North America
Segment reporting information
Revenues, net of interest expense									33,900		32,200		32,200
Operating Segments | Citicorp | EMEA
Segment reporting information
Revenues, net of interest expense									10,700		9,900		9,800
Operating Segments | Citicorp | Latin America
Segment reporting information
Revenues, net of interest expense									9,400		8,900		9,700
Operating Segments | Citicorp | Asia
Segment reporting information
Revenues, net of interest expense									14,400		13,700		13,900
Operating Segments | Global Consumer Banking
Segment reporting information
Revenues, net of interest expense									32,697		31,519		32,251
Provision (benefits) for income taxes									3,320		2,655		3,369
Income (loss) from continuing operations									3,893		4,954		6,214
Identifiable assets	$ 429,000				412,000				429,000		412,000
Provisions for credit losses and for benefits and claims									$ 7,600		6,400		5,500
Operating Segments | Institutional Clients Group
Segment reporting information
Number of countries where the entity provides a broad range of banking and financial products and services (over) | country	97								97
Revenues, net of interest expense									$ 35,667		33,227		33,332
Provision (benefits) for income taxes									7,008		4,260		4,173
Income (loss) from continuing operations									9,066		9,525		9,110
Identifiable assets	$ 1,336,000				1,277,000				1,336,000		1,277,000
Provisions for credit losses and for benefits and claims									15		486		962
Operating Segments | Corporate/Other
Segment reporting information
Revenues, net of interest expense									3,085		5,129		10,771
Provision (benefits) for income taxes									19,060		(471)		(102)
Income (loss) from continuing operations									(19,586)		554		2,062
Identifiable assets	$ 77,000				$ 103,000				77,000		103,000
Provisions for credit losses and for benefits and claims									$ 175		$ 69		$ 1,500

[1] Certain prior-period revenue and expense lines and totals were reclassified to conform to the current period’s presentation. See Note 3 to the Consolidated Financial Statements.