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SECURITIES - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
SECURITIES - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Debt Securities, Available-for-sale [Line Items]
Taxable interest income from securities	$ 167	$ 155	$ 500	$ 469
Impaired debt securities sold	0	0	0	0
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Securitizations of mortgage loans	32	38	87	82
Available-for-sale Securities
Debt Securities, Available-for-sale [Line Items]
Cumulative credit losses recognized in earnings	81	80	81	80	$ 81	$ 80	$ 79	$ 75
OTTI in earnings	1	1	3	6
Held-to-maturity Securities
Debt Securities, Available-for-sale [Line Items]
Cumulative credit losses recognized in earnings	$ 0	$ 0	$ 0	$ 0