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SECURITIES - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
SECURITIES - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Impaired debt securities sold	$ 0	$ 0	$ 0	$ 0
Taxable interest income from securities	155	146	469	432
Securities purchased under agreements to resell, gross	0		0			$ 0
Securities purchased under agreements to resell, offset	0		0			0
Securities purchased under agreements to resell	0		0			0
Securities sold under agreements to repurchase, gross	0		0			0
Securities sold under agreements to repurchase, offset	0		0			0
Securities sold under agreements to repurchase	0		0			0
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securitizations of mortgage loans	38	16	82	21
Available-for-sale Securities
Schedule of Available-for-sale Securities [Line Items]
Cumulative credit losses recognized in earnings	80	75	80	75	$ 79	$ 75	$ 73	$ 66
OTTI in earnings	1	3	6	11
Held-to-maturity Securities
Schedule of Available-for-sale Securities [Line Items]
Cumulative credit losses recognized in earnings	$ 0	$ 0	$ 0	0
Proprietary internal model \| Available-for-sale Securities
Schedule of Available-for-sale Securities [Line Items]
OTTI in earnings				$ 5