SECURITIES - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Securities, Available-for-sale [Line Items]
Taxable interest income from investment securities	$ 672	$ 625	$ 584
Impaired debt securities sold	0	0	0
Pretax non-credit related losses were deferred in OCI	4	0	27
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Securitizations of mortgage loans	142	134	68
Available-for-sale Securities
Debt Securities, Available-for-sale [Line Items]
Cumulative credit losses recognized in earnings	81	80	75	$ 66
Credit impairments recognized in earnings on securities that have been previously impaired	3	7	12
Held-to-maturity Securities
Debt Securities, Available-for-sale [Line Items]
Cumulative credit losses recognized in earnings	$ 0	$ 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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