ALLOWANCE FOR CREDIT LOSSES, NONACCRUAL LOANS AND LEASES, AND CONCENTRATIONS OF CREDIT RISK - Summary of Changes in Allowance for Credit Losses (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Apr. 06, 2022	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses, beginning of period		$ 1,964	$ 1,947	$ 1,758	$ 2,443
Allowance on PCD loans and leases at acquisition		0		101
Charge-offs		(116)	(87)	(308)	(439)
Recoveries		42	43	126	159
Net charge-offs		(74)	(44)	(182)	(280)
Provision expense (benefit) for loans and leases		90	(48)	303	(308)
Allowance for loan and lease losses, end of period		1,980	1,855	1,980	1,855
Unfunded lending commitments [Roll Forward]
Allowance for unfunded lending commitments, beginning of period		183	134	176	227
Provision expense (benefit) for unfunded lending commitments		33	15	39	(78)
Allowance on PCD unfunded lending commitments at acquisition		0		1
Allowance for unfunded lending commitments, end of period		216	149	216	149
Total allowance for credit losses, end of period		2,196	2,004	2,196	2,004
Provision expense (benefit) for loans and leases		90	(48)	303	(308)
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
Unfunded lending commitments [Roll Forward]
Provision expense (benefit) for loans and leases				169
Investors Acquisition
Allowance for Loan and Lease Losses [Roll Forward]
Allowance on PCD loans and leases at acquisition	$ 101
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses, beginning of period		987	953	821	1,233
Allowance on PCD loans and leases at acquisition		0		99
Charge-offs		(22)	(17)	(49)	(196)
Recoveries		7	3	13	37
Net charge-offs		(15)	(14)	(36)	(159)
Provision expense (benefit) for loans and leases		58	(72)	146	(207)
Allowance for loan and lease losses, end of period		1,030	867	1,030	867
Unfunded lending commitments [Roll Forward]
Allowance for unfunded lending commitments, beginning of period		166	121	153	186
Provision expense (benefit) for unfunded lending commitments		6	9	18	(56)
Allowance on PCD unfunded lending commitments at acquisition		0		1
Allowance for unfunded lending commitments, end of period		172	130	172	130
Total allowance for credit losses, end of period		1,202	997	1,202	997
Provision expense (benefit) for loans and leases		58	(72)	146	(207)
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		977	994	937	1,210
Allowance on PCD loans and leases at acquisition		0		2
Charge-offs		(94)	(70)	(259)	(243)
Recoveries		35	40	113	122
Net charge-offs		(59)	(30)	(146)	(121)
Provision expense (benefit) for loans and leases		32	24	157	(101)
Allowance for loan and lease losses, end of period		950	988	950	988
Unfunded lending commitments [Roll Forward]
Allowance for unfunded lending commitments, beginning of period		17	13	23	41
Provision expense (benefit) for unfunded lending commitments		27	6	21	(22)
Allowance on PCD unfunded lending commitments at acquisition		0		0
Allowance for unfunded lending commitments, end of period		44	19	44	19
Total allowance for credit losses, end of period		994	1,007	994	1,007
Provision expense (benefit) for loans and leases		$ 32	$ 24	$ 157	$ (101)