GOODWILL AND INTANGIBLE ASSETS - Goodwill by Reporting Units (Details) - USD ($) $ in Millions	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016
Goodwill:
Goodwill	$ 22,349	$ 22,265	$ 21,659	$ 22,349
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	148.00%
Operating Segments \| Global Consumer Banking \| Reporting units \| Asia Global Consumer Banking
Goodwill:
Goodwill	$ 4,900
Fair value of goodwill as a percentage of allocated book value	157.00%
Operating Segments \| Global Consumer Banking \| Reporting units \| Latin America Global Consumer Banking
Goodwill:
Goodwill	$ 1,178
Fair value of goodwill as a percentage of allocated book value	180.00%
Operating Segments \| Institutional Clients Group \| Reporting units \| ICG—Banking
Goodwill:
Goodwill	$ 2,998
Fair value of goodwill as a percentage of allocated book value	194.00%
Operating Segments \| Institutional Clients Group \| Reporting units \| ICG—Markets and Securities Services
Goodwill:
Goodwill	$ 6,525
Fair value of goodwill as a percentage of allocated book value	115.00%
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%