FAIR VALUE MEASUREMENT - Estimate Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Assets
Loans	$ 7,707		$ 6,854
Liabilities
Deposits	1,310,281		1,280,671
Purchased credit-deteriorated	19,238	$ 21,638	24,956	$ 26,298	$ 20,380	$ 12,783
Loans, net of unearned income	676,834		675,883
Corporate
Assets
Loans	7,693		6,840
Liabilities
Purchased credit-deteriorated	3,127	$ 4,084	5,402	$ 6,824	$ 3,451	$ 2,886
Loans, net of unearned income	400,939		387,044
Carrying value
Assets
Investments, net of allowance	181,900		110,300
Securities borrowed and purchased under agreements to resell	121,200		109,500
Loans	649,300		643,300
Other financial assets	397,900		383,200
Liabilities
Deposits	1,307,500		1,278,700
Securities loaned and sold under agreements to repurchase	145,100		139,300
Long-term debt	188,200		204,600
Other financial liabilities	114,800		102,400
Purchased credit-deteriorated	19,200		25,000
Fair value
Assets
Investments, net of allowance	183,400		113,200
Securities borrowed and purchased under agreements to resell	121,200		109,500
Loans	665,200		663,900
Other financial assets	397,900		383,200
Liabilities
Deposits	1,308,300		1,278,800
Securities loaned and sold under agreements to repurchase	145,100		139,300
Long-term debt	203,100		221,200
Other financial liabilities	114,800		102,400
Fair value | Corporate
Liabilities
Loans, net of unearned income	7,693		6,840
Fair value | Level 1
Assets
Investments, net of allowance	73,800		23,300
Securities borrowed and purchased under agreements to resell	0		0
Loans	0		0
Other financial assets	279,300		291,500
Liabilities
Deposits	0		0
Securities loaned and sold under agreements to repurchase	0		0
Long-term debt	0		0
Other financial liabilities	0		0
Fair value | Level 2
Assets
Investments, net of allowance	106,900		87,000
Securities borrowed and purchased under agreements to resell	120,900		109,500
Loans	0		600
Other financial assets	19,900		18,100
Liabilities
Deposits	1,138,600		1,093,300
Securities loaned and sold under agreements to repurchase	145,000		139,300
Long-term debt	184,000		197,800
Other financial liabilities	19,600		19,200
Fair value | Level 3
Assets
Investments, net of allowance	2,700		2,900
Securities borrowed and purchased under agreements to resell	300		0
Loans	665,200		663,300
Other financial assets	98,700		73,600
Liabilities
Deposits	169,700		185,500
Securities loaned and sold under agreements to repurchase	100		0
Long-term debt	19,100		23,400
Other financial liabilities	95,200		83,200
Fair value | Level 3 | Corporate
Fair value measurements additional disclosures
Unfunded lending commitments	6,700		7,300
Lease financing
Liabilities
Loans, net of unearned income	600		700
Lease financing | Corporate
Liabilities
Loans, net of unearned income	$ 602		$ 738