CREDIT QUALITY AND THE ALLOWANCE FOR CREDIT LOSSES - Summary of Changes in Allowance for Credit Losses (Details) - USD ($) $ in Millions		12 Months Ended
	Apr. 06, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses, beginning of period		$ 1,983	$ 1,758	$ 2,443
Allowance on PCD loans and leases at acquisition			101
Charge-offs		(757)	(434)	(539)
Recoveries		148	164	214
Net charge-offs		(609)	(270)	(325)
Provision expense (benefit) for loans and leases		724	394	(360)
Allowance for loan and lease losses, end of period		2,098	1,983	1,758
Unfunded lending commitments [Roll Forward]
Allowance for unfunded lending commitments, beginning of period		257	176	227
Provision expense (benefit) for unfunded lending commitments		(37)	80	(51)
Allowance on PCD unfunded lending commitments at acquisition			1
Allowance for unfunded lending commitments, end of period		220	257	176
Total allowance for credit losses, end of period		2,318	2,240	1,934
Investors Acquisition
Allowance for Loan and Lease Losses [Roll Forward]
Allowance on PCD loans and leases at acquisition	$ 101
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		1,060	821	1,233
Allowance on PCD loans and leases at acquisition			99
Charge-offs		(285)	(70)	(218)
Recoveries		18	18	54
Net charge-offs		(267)	(52)	(164)
Provision expense (benefit) for loans and leases		457	192	(248)
Allowance for loan and lease losses, end of period		1,250	1,060	821
Unfunded lending commitments [Roll Forward]
Allowance for unfunded lending commitments, beginning of period		207	153	186
Provision expense (benefit) for unfunded lending commitments		(32)	53	(33)
Allowance on PCD unfunded lending commitments at acquisition			1
Allowance for unfunded lending commitments, end of period		175	207	153
Total allowance for credit losses, end of period		1,425	1,267	974
Commercial | Investors Acquisition
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs			(34)
Retail
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses, beginning of period		923	937	1,210
Allowance on PCD loans and leases at acquisition			2
Charge-offs		(472)	(364)	(321)
Recoveries		130	146	160
Net charge-offs		(342)	(218)	(161)
Provision expense (benefit) for loans and leases		267	202	(112)
Allowance for loan and lease losses, end of period		848	923	937
Unfunded lending commitments [Roll Forward]
Allowance for unfunded lending commitments, beginning of period		50	23	41
Provision expense (benefit) for unfunded lending commitments		(5)	27	(18)
Allowance on PCD unfunded lending commitments at acquisition			0
Allowance for unfunded lending commitments, end of period		45	50	23
Total allowance for credit losses, end of period		$ 893	$ 973	$ 960