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SECURITIES - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
SECURITIES - Narrative (Details) - USD ($)	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Taxable interest income from securities	$ 164,000,000	$ 165,000,000	$ 330,000,000	$ 333,000,000
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Securitizations of mortgage loans	13,000,000	29,000,000	44,000,000	55,000,000
Available-for-sale Securities
Debt Securities, Available-for-sale [Line Items]
Cumulative credit losses recognized in earnings	57,000,000	81,000,000	57,000,000	81,000,000	$ 78,000,000	$ 81,000,000	$ 80,000,000	$ 80,000,000
OTTI in earnings	0	1,000,000	1,000,000	2,000,000
Held-to-maturity Securities
Debt Securities, Available-for-sale [Line Items]
Cumulative credit losses recognized in earnings	$ 0	$ 0	$ 0	$ 0