SECURITIES - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Impaired debt securities sold	$ 0	$ 0
Pretax non-credit related losses were deferred in OCI	19	40
Taxable interest income from securities	160	145
Securities purchased under agreements to resell, gross	0		$ 0
Securities purchased under agreements to resell, offset	0		0
Securities purchased under agreements to resell	0		0
Securities sold under agreements to repurchase, gross	0		0
Securities sold under agreements to repurchase, offset	0		0
Securities sold under agreements to repurchase	0		0
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securitizations of mortgage loans	22	0
Available-for-sale Securities
Schedule of Available-for-sale Securities [Line Items]
Cumulative credit losses recognized in earnings	75	66	$ 75	$ 66
Credit impairments recognized in earnings on securities that have been previously impaired	1	1
Held-to-maturity Securities
Schedule of Available-for-sale Securities [Line Items]
Cumulative credit losses recognized in earnings	$ 0	$ 0

Amount of operating interest income, including amortization and accretion of premiums and discounts, on securities subject to state, federal and other income tax.

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The cash inflow from securitizing loans held with the intention to resell in the near future.

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