FAIR VALUE MEASUREMENT - Estimate Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Assets
Loans	$ 7,411		$ 5,360
Liabilities
Deposits	1,273,506		1,365,954
Allowance for credit losses	17,629	$ 17,496	16,974	$ 16,309	$ 15,952	$ 16,455
Loans, net of unearned income	666,348		657,221
Corporate loans
Assets
Loans	7,189		5,123
Liabilities
Allowance for credit losses	2,717	$ 2,630	2,855	$ 2,948	$ 2,969	$ 2,415
Loans, net of unearned income	288,634		289,154
Carrying value
Assets
HTM debt securities, net of allowance(1)	264,900		274,300
Securities borrowed and purchased under agreements to resell	128,900		125,900
Loans	641,100		634,500
Other financial assets	352,400		427,100
Liabilities
Deposits	1,270,800		1,364,100
Securities loaned and sold under agreements to repurchase	196,100		131,600
Long-term debt	163,100		165,600
Other financial liabilities	144,800		142,400
Allowance for credit losses	17,600		17,000
Estimated fair value
Assets
HTM debt securities, net of allowance(1)	236,800		249,200
Securities borrowed and purchased under agreements to resell	129,000		125,900
Loans	646,000		634,900
Other financial assets	352,400		427,100
Liabilities
Deposits	1,270,100		1,345,400
Securities loaned and sold under agreements to repurchase	196,100		131,600
Long-term debt	162,200		160,500
Other financial liabilities	144,800		142,400
Estimated fair value | Corporate loans
Liabilities
Loans, net of unearned income	7,189		5,123
Estimated fair value | Level 1
Assets
HTM debt securities, net of allowance(1)	123,700		123,200
Securities borrowed and purchased under agreements to resell	0		0
Loans	0		0
Other financial assets	236,200		320,000
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)	110,500		123,100
Securities borrowed and purchased under agreements to resell	129,000		125,900
Loans	0		0
Other financial assets	17,800		22,000
Liabilities
Deposits	1,078,500		1,159,400
Securities loaned and sold under agreements to repurchase	196,100		131,600
Long-term debt	153,500		151,100
Other financial liabilities	34,200		26,500
Estimated fair value | Level 3
Assets
HTM debt securities, net of allowance(1)	2,600		2,900
Securities borrowed and purchased under agreements to resell	0		0
Loans	646,000		634,900
Other financial assets	98,400		85,100
Liabilities
Deposits	191,600		186,000
Securities loaned and sold under agreements to repurchase	0		0
Long-term debt	8,700		9,400
Other financial liabilities	110,600		115,900
Estimated fair value | Level 3 | Corporate loans
Fair value measurements additional disclosures
Unfunded lending commitments	8,300		13,700
Lease financing
Liabilities
Loans, net of unearned income	$ 300		400
Lease financing | Corporate loans
Liabilities
Loans, net of unearned income			$ 354