DISCONTINUED OPERATIONS AND SIGNIFICANT DISPOSALS - Significant Disposals (Details) loan in Thousands, customer_account in Thousands, $ in Millions				3 Months Ended		12 Months Ended
		Jan. 01, 2016	Nov. 15, 2015 USD ($) employee branch customer_account	Mar. 31, 2017 USD ($) employee branch	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) loan	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2018 USD ($)	Aug. 31, 2017 USD ($)	Dec. 14, 2015 USD ($) employee customer_account	Nov. 01, 2015 USD ($) employee branch customer_account
Discontinued operations
Pre-tax gain (loss) on sale	[1]					$ 602	$ 404	$ 3,210
Mexico Asset Management Business | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Discontinued operations
Income (loss) before taxes						164	155	159
Fixed Income Analytics and Index Businesses | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Discontinued operations
Goodwill										$ 72
Income (loss) before taxes						31	55	54
Assets										112
Liabilities										$ 18
Pre-tax gain (loss) on sale						580
After tax gain (loss) on sale of disposal						355
US Mortgage Servicing Operations | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Discontinued operations
Income (loss) before taxes						0	0	0
Pre-tax gain (loss) on sale						(331)
After tax gain (loss) on sale of disposal						(207)
Loss on disposal						382
Derecognition of servicing-related assets						1,162
Derecognition of mortgage servicing rights						$ 1,046
Number of loans to be disposed of | loan						750
Loans, net						$ 93,000
CitiFinancial Canada | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Discontinued operations
Income (loss) before taxes						41	139	118
Assets				$ 1,900
Liabilities				1,500
Pre-tax gain (loss) on sale						350
After tax gain (loss) on sale of disposal						178
Loans, net				$ 1,700
Number of branches | branch				220
Number of employees | employee				1,400
Debt settled				$ 400
Novation of 80% Primerica Coinsurance Agreement | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Discontinued operations
Income (loss) before taxes						0	0	135
Liabilities							2,700
Pre-tax gain (loss) on sale							404
After tax gain (loss) on sale of disposal							263
Coinsurance agreement percentage		80.00%
Derecognition of available for sale securities and cash							1,500
Derecognition of deferred acquisition costs							950
OneMain Financial Business | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Discontinued operations
Income (loss) before taxes						0	0	663
Assets			$ 10,200
Liabilities			8,400
Pre-tax gain (loss) on sale			2,600
After tax gain (loss) on sale of disposal			1,600
Loans, net			$ 7,800
Number of branches | branch			1,100
Number of employees | employee			5,500
Number of customer accounts | customer_account			1,300
Investments			$ 1,400
Long-term debt			6,200
Short-term borrowings			$ 1,100
Gain on disposal including redemption of long term debt, net of tax					$ 800
Japan Cards Business | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Discontinued operations
Income (loss) before taxes						0	0	(5)
Pre-tax gain (loss) on sale								180
After tax gain (loss) on sale of disposal								155
Loans, net											$ 1,350
Number of employees | employee											840
Number of customer accounts | customer_account											720
Japan Retail Business | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Discontinued operations
Income (loss) before taxes						$ 0	$ 0	(57)
Pre-tax gain (loss) on sale								446
After tax gain (loss) on sale of disposal								$ 276
Loans, net												$ 563
Number of branches | branch												32
Number of employees | employee												1,600
Number of customer accounts | customer_account												725
Deposits												$ 20,000
Forecast | Mexico Asset Management Business | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Discontinued operations
Net book value derecognized									$ 72
Goodwill									$ 32

[1] See Note 2 to the Consolidated Financial Statements for further information on significant disposals.