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SECURITIES - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
SECURITIES - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Taxable interest income from securities	$ 166	$ 168
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Securitizations of mortgage loans	31	26
Available-for-sale Securities
Debt Securities, Available-for-sale [Line Items]
Cumulative credit losses recognized in earnings	78	80	$ 81	$ 80
OTTI in earnings	1	1
Held-to-maturity Securities
Debt Securities, Available-for-sale [Line Items]
Cumulative credit losses recognized in earnings	$ 0	$ 0