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GOODWILL AND INTANGIBLE ASSETS - Narrative (Details)		3 Months Ended	12 Months Ended
GOODWILL AND INTANGIBLE ASSETS - Narrative (Details)	Mar. 04, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 USD ($) reportingUnit	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Goodwill [Line Items]
Number of reporting units \| reportingUnit			2
Goodwill impairment		$ 0	$ 0	$ 0	$ 0
Amortization of intangibles			11,000,000	11,000,000	$ 3,000,000
Consumer Banking
Goodwill [Line Items]
Goodwill accumulated impairment loss		5,900,000,000	5,900,000,000	5,900,000,000
Commercial Banking
Goodwill [Line Items]
Goodwill acquired during period	$ 6,000,000
Goodwill accumulated impairment loss		$ 50,000,000	$ 50,000,000	$ 50,000,000