GOODWILL AND INTANGIBLE ASSETS - Goodwill by Reporting Units (Details) - USD ($) $ in Millions	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Goodwill:
Goodwill	$ 22,345	$ 22,349	$ 22,265	$ 21,659
Operating Segments | Global Consumer Banking | Reporting units | North America Global Consumer Banking
Goodwill:
Goodwill	$ 6,732
Fair value of goodwill as a percentage of allocated book value	157.00%
Operating Segments | Global Consumer Banking | Reporting units | Asia Global Consumer Banking
Goodwill:
Goodwill	$ 4,893
Fair value of goodwill as a percentage of allocated book value	143.00%
Operating Segments | Global Consumer Banking | Reporting units | Latin America Global Consumer Banking
Goodwill:
Goodwill	$ 1,174
Fair value of goodwill as a percentage of allocated book value	191.00%
Operating Segments | Institutional Clients Group | Reporting units | ICG—Banking
Goodwill:
Goodwill	$ 2,986
Fair value of goodwill as a percentage of allocated book value	268.00%
Operating Segments | Institutional Clients Group | Reporting units | ICG—Markets and Securities Services
Goodwill:
Goodwill	$ 6,544
Fair value of goodwill as a percentage of allocated book value	132.00%
Operating Segments | Citi Holdings | Reporting units | Citi Holdings—Consumer Latin America
Goodwill:
Goodwill	$ 16
Fair value of goodwill as a percentage of allocated book value	103.00%