INVESTMENT SECURITIES - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Pre-tax, other-than-temporary impairments on investment securities recognized in earnings	$ 8	$ 31	$ 64
Gross realized gains on available-for-sale investment securities	244	61	128
Gross realized losses	(24)	(55)	(87)
Proceeds from investment securities sales and early redemptions by issuers	3,241	1,718	$ 5,746
Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value of investments	$ 4,413	$ 4,406