FAIR VALUE MEASUREMENT - Estimate Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Assets
Loans	$ 7,159		$ 6,854
Liabilities
Deposits	1,347,528		1,280,671
Purchased credit-deteriorated	17,715	$ 19,238	24,956	$ 26,426	$ 26,298	$ 12,783
Loans, net of unearned income	664,764		675,883
Corporate
Assets
Loans	7,146		6,840
Liabilities
Purchased credit-deteriorated	3,047	$ 3,127	5,402	$ 6,938	$ 6,824	$ 2,886
Loans, net of unearned income	400,514		387,044
Carrying value
Assets
Investments, net of allowance	203,000		110,300
Securities borrowed and purchased under agreements to resell	125,500		109,500
Loans	639,400		643,300
Other financial assets	421,000		383,200
Liabilities
Deposits	1,344,800		1,278,700
Securities loaned and sold under agreements to repurchase	133,900		139,300
Long-term debt	180,100		204,600
Other financial liabilities	115,400		102,400
Purchased credit-deteriorated	17,700		25,000
Estimated fair value
Assets
Investments, net of allowance	203,700		113,200
Securities borrowed and purchased under agreements to resell	125,500		109,500
Loans	654,200		663,900
Other financial assets	421,000		383,200
Liabilities
Deposits	1,345,500		1,278,800
Securities loaned and sold under agreements to repurchase	133,900		139,300
Long-term debt	194,000		221,200
Other financial liabilities	115,400		102,400
Estimated fair value | Corporate
Liabilities
Loans, net of unearned income	7,146		6,840
Estimated fair value | Level 1
Assets
Investments, net of allowance	89,100		23,300
Securities borrowed and purchased under agreements to resell	0		0
Loans	0		0
Other financial assets	302,500		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
Estimated fair value | Level 2
Assets
Investments, net of allowance	111,600		87,000
Securities borrowed and purchased under agreements to resell	121,800		109,500
Loans	0		600
Other financial assets	21,300		18,100
Liabilities
Deposits	1,178,000		1,093,300
Securities loaned and sold under agreements to repurchase	133,600		139,300
Long-term debt	179,800		197,800
Other financial liabilities	18,000		19,200
Estimated fair value | Level 3
Assets
Investments, net of allowance	3,000		2,900
Securities borrowed and purchased under agreements to resell	3,700		0
Loans	654,200		663,300
Other financial assets	97,200		73,600
Liabilities
Deposits	167,500		185,500
Securities loaned and sold under agreements to repurchase	300		0
Long-term debt	14,200		23,400
Other financial liabilities	97,400		83,200
Estimated fair value | Level 3 | Corporate
Fair value measurements additional disclosures
Unfunded lending commitments	9,100		7,300
Lease financing
Liabilities
Loans, net of unearned income	500		700
Lease financing | Corporate
Liabilities
Loans, net of unearned income	$ 472		$ 738