FAIR VALUE MEASUREMENT - Estimate Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Assets
Loans	$ 8,854	$ 7,594
Liabilities
Deposits	1,307,163	1,308,681
Allowance for credit losses	18,296	18,145	$ 17,169	$ 16,974
Loans, net of unearned income	674,578	689,362
Corporate loans
Assets
Loans	8,551	7,281
Liabilities
Allowance for credit losses	2,772	2,714	$ 2,780	$ 2,855
Loans, net of unearned income	292,819	300,165
Carrying value
Assets
HTM debt securities, net of allowance(1)	257,800	259,700
Securities borrowed and purchased under agreements to resell	150,700	139,600
Loans	647,200	663,300
Other financial assets	366,700	347,500
Liabilities
Deposits	1,304,300	1,306,200
Securities loaned and sold under agreements to repurchase	225,600	215,600
Long-term debt	170,100	170,300
Other financial liabilities	137,800	132,800
Allowance for credit losses	18,300	18,100
Estimated fair value
Assets
HTM debt securities, net of allowance(1)	236,800	240,600
Securities borrowed and purchased under agreements to resell	150,700	139,700
Loans	649,700	673,200
Other financial assets	366,700	347,500
Liabilities
Deposits	1,304,100	1,305,900
Securities loaned and sold under agreements to repurchase	225,600	215,600
Long-term debt	173,600	173,400
Other financial liabilities	137,800	132,800
Estimated fair value | Corporate loans
Liabilities
Loans, net of unearned income	8,551	7,281
Estimated fair value | Level 1
Assets
HTM debt securities, net of allowance(1)	123,700	124,000
Securities borrowed and purchased under agreements to resell	0	0
Loans	0	0
Other financial assets	255,700	243,100
Liabilities
Deposits	0	0
Securities loaned and sold under agreements to repurchase	0	0
Long-term debt	0	0
Other financial liabilities	0	0
Estimated fair value | Level 2
Assets
HTM debt securities, net of allowance(1)	110,700	114,100
Securities borrowed and purchased under agreements to resell	150,700	139,700
Loans	0	0
Other financial assets	17,000	17,800
Liabilities
Deposits	1,106,700	1,116,500
Securities loaned and sold under agreements to repurchase	225,600	215,600
Long-term debt	170,000	168,000
Other financial liabilities	22,300	29,200
Estimated fair value | Level 3
Assets
HTM debt securities, net of allowance(1)	2,400	2,500
Securities borrowed and purchased under agreements to resell	0	0
Loans	649,700	673,200
Other financial assets	94,000	86,600
Liabilities
Deposits	197,400	189,400
Securities loaned and sold under agreements to repurchase	0	0
Long-term debt	3,600	5,400
Other financial liabilities	115,500	103,600
Estimated fair value | Level 3 | Corporate loans
Fair value measurements additional disclosures
Unfunded lending commitments	12,900	14,200
Lease financing
Liabilities
Loans, net of unearned income	$ 300	300
Lease financing | Corporate loans
Liabilities
Loans, net of unearned income		$ 275