FAIR VALUE MEASUREMENT - Estimate Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Assets
Loans	$ 7,525	$ 6,854
Liabilities
Deposits	1,300,975	1,280,671
Purchased credit-deteriorated	21,638	24,956	$ 20,380	$ 12,783
Loans, net of unearned income	665,988	675,883
Corporate
Assets
Loans	7,510	6,840
Liabilities
Purchased credit-deteriorated	4,084	5,402	$ 3,451	$ 2,886
Loans, net of unearned income	391,954	387,044
Carrying value
Assets
Investments	167,100	110,300
Securities borrowed and purchased under agreements to resell	116,200	109,500
Loans	636,200	643,300
Other financial assets	415,800	383,200
Liabilities
Deposits	1,297,800	1,278,700
Securities loaned and sold under agreements to repurchase	150,500	139,300
Long-term debt	188,300	204,600
Other financial liabilities	117,600	102,400
Purchased credit-deteriorated	21,600	25,000
Fair value
Assets
Investments	167,300	113,200
Securities borrowed and purchased under agreements to resell	116,200	109,500
Loans	653,100	663,900
Other financial assets	415,800	383,200
Liabilities
Deposits	1,298,600	1,278,800
Securities loaned and sold under agreements to repurchase	150,500	139,300
Long-term debt	201,400	221,200
Other financial liabilities	117,600	102,400
Fair value | Corporate
Liabilities
Loans, net of unearned income	7,510	6,840
Fair value | Level 1
Assets
Investments	59,400	23,300
Securities borrowed and purchased under agreements to resell	0	0
Loans	0	0
Other financial assets	305,500	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
Fair value | Level 2
Assets
Investments	105,400	87,000
Securities borrowed and purchased under agreements to resell	116,200	109,500
Loans	900	600
Other financial assets	19,200	18,100
Liabilities
Deposits	1,125,700	1,093,300
Securities loaned and sold under agreements to repurchase	150,500	139,300
Long-term debt	181,000	197,800
Other financial liabilities	18,300	19,200
Fair value | Level 3
Assets
Investments	2,500	2,900
Securities borrowed and purchased under agreements to resell	0	0
Loans	652,200	663,300
Other financial assets	91,100	73,600
Liabilities
Deposits	172,900	185,500
Securities loaned and sold under agreements to repurchase	0	0
Long-term debt	20,400	23,400
Other financial liabilities	99,300	83,200
Fair value | Level 3 | Corporate
Fair value measurements additional disclosures
Unfunded lending commitments	7,300	7,300
Lease financing
Liabilities
Loans, net of unearned income	600	700
Lease financing | Corporate
Liabilities
Loans, net of unearned income	$ 595	$ 738