CREDIT QUALITY AND THE ALLOWANCE FOR CREDIT LOSSES (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses, beginning of period	$ 2,044	$ 1,964	$ 1,983	$ 1,758
Allowance on PCD loans and leases at acquisition		0		101
Charge-offs	(191)	(116)	(551)	(308)
Recoveries	38	42	113	126
Net charge-offs	(153)	(74)	(438)	(182)
Provision expense (benefit) for loans and leases	189	90	535	303
Allowance for loan and lease losses, end of period	2,080	1,980	2,080	1,980
Unfunded lending commitments [Roll Forward]
Allowance for unfunded lending commitments, beginning of period	255	183	257	176
Provision expense (benefit) for unfunded lending commitments	(17)	33	(19)	39
Allowance on PCD unfunded lending commitments at acquisition		0		1
Allowance for unfunded lending commitments, end of period	238	216	238	216
Total allowance for credit losses, end of period	2,318	2,196	2,318	2,196
Online Deposit Business And 80 East Coast Branches Of HSBC Bank And Investors
Allowance for Loan and Lease Losses [Roll Forward]
Provision expense (benefit) for loans and leases				169
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses, beginning of period	1,157	987	1,060	821
Allowance on PCD loans and leases at acquisition		0		99
Charge-offs	(74)	(22)	(212)	(49)
Recoveries	4	7	14	13
Net charge-offs	(70)	(15)	(198)	(36)
Provision expense (benefit) for loans and leases	146	58	371	146
Allowance for loan and lease losses, end of period	1,233	1,030	1,233	1,030
Unfunded lending commitments [Roll Forward]
Allowance for unfunded lending commitments, beginning of period	213	166	207	153
Provision expense (benefit) for unfunded lending commitments	(21)	6	(15)	18
Allowance on PCD unfunded lending commitments at acquisition		0		1
Allowance for unfunded lending commitments, end of period	192	172	192	172
Total allowance for credit losses, end of period	1,425	1,202	1,425	1,202
Commercial | Investors Acquisition
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs				(33)
Retail
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses, beginning of period	887	977	923	937
Allowance on PCD loans and leases at acquisition		0		2
Charge-offs	(117)	(94)	(339)	(259)
Recoveries	34	35	99	113
Net charge-offs	(83)	(59)	(240)	(146)
Provision expense (benefit) for loans and leases	43	32	164	157
Allowance for loan and lease losses, end of period	847	950	847	950
Unfunded lending commitments [Roll Forward]
Allowance for unfunded lending commitments, beginning of period	42	17	50	23
Provision expense (benefit) for unfunded lending commitments	4	27	(4)	21
Allowance on PCD unfunded lending commitments at acquisition		0		0
Allowance for unfunded lending commitments, end of period	46	44	46	44
Total allowance for credit losses, end of period	$ 893	$ 994	$ 893	$ 994