CREDIT QUALITY AND THE ALLOWANCE FOR CREDIT LOSSES - Summary of Changes in Allowance for Credit Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses, beginning of period	$ 2,098	$ 1,983	$ 1,758
Allowance on PCD loans and leases at acquisition			101
Charge-offs	(923)	(757)	(434)
Recoveries	177	148	164
Net charge-offs	(746)	(609)	(270)
Provision expense (benefit) for loans and leases	709	724	394
Allowance for loan and lease losses, end of period	2,061	2,098	1,983
Unfunded lending commitments [Roll Forward]
Allowance for unfunded lending commitments, beginning of period	220	257	176
Provision expense (benefit) for unfunded lending commitments	(22)	(37)	80
Allowance on PCD unfunded lending commitments at acquisition			1
Allowance for unfunded lending commitments, end of period	198	220	257
Total allowance for credit losses, end of period	2,259	2,318	2,240
Investors Acquisition
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs			(34)
HSBC Bank And Investors Acquisitions
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	1,250	1,060	821
Allowance on PCD loans and leases at acquisition			99
Charge-offs	(419)	(285)	(70)
Recoveries	43	18	18
Net charge-offs	(376)	(267)	(52)
Provision expense (benefit) for loans and leases	266	457	192
Allowance for loan and lease losses, end of period	1,140	1,250	1,060
Unfunded lending commitments [Roll Forward]
Allowance for unfunded lending commitments, beginning of period	175	207	153
Provision expense (benefit) for unfunded lending commitments	(20)	(32)	53
Allowance on PCD unfunded lending commitments at acquisition			1
Allowance for unfunded lending commitments, end of period	155	175	207
Total allowance for credit losses, end of period	1,295	1,425	1,267
Retail
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses, beginning of period	848	923	937
Allowance on PCD loans and leases at acquisition			2
Charge-offs	(504)	(472)	(364)
Recoveries	134	130	146
Net charge-offs	(370)	(342)	(218)
Provision expense (benefit) for loans and leases	443	267	202
Allowance for loan and lease losses, end of period	921	848	923
Unfunded lending commitments [Roll Forward]
Allowance for unfunded lending commitments, beginning of period	45	50	23
Provision expense (benefit) for unfunded lending commitments	(2)	(5)	27
Allowance on PCD unfunded lending commitments at acquisition			0
Allowance for unfunded lending commitments, end of period	43	45	50
Total allowance for credit losses, end of period	$ 964	$ 893	$ 973