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		6 Months Ended
	Apr. 06, 2022	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses, beginning of period		$ 1,720	$ 2,194	$ 1,758	$ 2,443
Allowance on PCD loans and leases at acquisition		101		101
Charge-offs		(91)	(125)	(192)	(352)
Recoveries		42	47	84	116
Net charge-offs		(49)	(78)	(108)	(236)
Provision expense (benefit) for loans and leases		192	(169)	213	(260)
Allowance for loan and lease losses, end of period		1,964	1,947	1,964	1,947
Unfunded lending commitments [Roll Forward]
Allowance for unfunded lending commitments, beginning of period		158	178	176	227
Provision expense (benefit) for unfunded lending commitments		24	(44)	6	(93)
Allowance on PCD unfunded lending commitments at acquisition		1		1
Allowance for unfunded lending commitments, end of period		183	134	183	134
Total allowance for credit losses, end of period		2,147	2,081	2,147	2,081
Provision expense (benefit) for loans and leases		192	(169)	213	(260)
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		145		169
Unfunded lending commitments [Roll Forward]
Provision expense (benefit) for loans and leases		145		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		778	1,146	821	1,233
Allowance on PCD loans and leases at acquisition		99		99
Charge-offs		(13)	(45)	(27)	(179)
Recoveries		3	4	6	34
Net charge-offs		(10)	(41)	(21)	(145)
Provision expense (benefit) for loans and leases		120	(152)	88	(135)
Allowance for loan and lease losses, end of period		987	953	987	953
Unfunded lending commitments [Roll Forward]
Allowance for unfunded lending commitments, beginning of period		147	165	153	186
Provision expense (benefit) for unfunded lending commitments		18	(44)	12	(65)
Allowance on PCD unfunded lending commitments at acquisition		1		1
Allowance for unfunded lending commitments, end of period		166	121	166	121
Total allowance for credit losses, end of period		1,153	1,074	1,153	1,074
Provision expense (benefit) for loans and leases		120	(152)	88	(135)
Commercial | Investors Acquisition
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs		(33)		(33)
Retail
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses, beginning of period		942	1,048	937	1,210
Allowance on PCD loans and leases at acquisition		2		2
Charge-offs		(78)	(80)	(165)	(173)
Recoveries		39	43	78	82
Net charge-offs		(39)	(37)	(87)	(91)
Provision expense (benefit) for loans and leases		72	(17)	125	(125)
Allowance for loan and lease losses, end of period		977	994	977	994
Unfunded lending commitments [Roll Forward]
Allowance for unfunded lending commitments, beginning of period		11	13	23	41
Provision expense (benefit) for unfunded lending commitments		6	0	(6)	(28)
Allowance on PCD unfunded lending commitments at acquisition		0		0
Allowance for unfunded lending commitments, end of period		17	13	17	13
Total allowance for credit losses, end of period		994	1,007	994	1,007
Provision expense (benefit) for loans and leases		$ 72	$ (17)	$ 125	$ (125)