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SECURITIES - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
SECURITIES - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Taxable interest income from securities	$ 153	$ 167	$ 483	$ 500
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Securitizations of mortgage loans	28	32	72	87
Available-for-sale Securities
Debt Securities, Available-for-sale [Line Items]
Cumulative credit losses recognized in earnings	57	81	57	81	$ 57	$ 81	$ 81	$ 80
OTTI in earnings	(1)	(1)	(2)	(3)
Held-to-maturity Securities
Debt Securities, Available-for-sale [Line Items]
Cumulative credit losses recognized in earnings	$ 0	$ 0	$ 0	$ 0