FAIR VALUE MEASUREMENT - Estimate Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Assets
Loans	$ 3,999	$ 4,085
Liabilities
Deposits	1,184,911	1,070,590
Purchased credit-deteriorated	20,841	12,783	$ 12,329	$ 12,315
Loans, net of unearned income	721,020	699,483
Corporate
Assets
Loans	3,981	4,067
Liabilities
Purchased credit-deteriorated	3,451	2,886	$ 2,731	$ 2,811
Loans, net of unearned income	432,590	389,935
Carrying value
Assets
Investments	88,700	86,400
Securities borrowed and purchased under agreements to resell	106,900	98,100
Loans	695,100	681,200
Other financial assets	386,900	262,400
Liabilities
Deposits	1,182,300	1,068,300
Securities loaned and sold under agreements to repurchase	159,600	125,700
Long-term debt	213,200	193,000
Other financial liabilities	145,600	110,200
Purchased credit-deteriorated	20,800	12,800
Fair value
Assets
Investments	90,000	87,800
Securities borrowed and purchased under agreements to resell	106,900	98,100
Loans	711,200	677,700
Other financial assets	386,900	262,400
Liabilities
Deposits	1,182,300	1,066,700
Securities loaned and sold under agreements to repurchase	159,600	125,700
Long-term debt	214,800	203,800
Other financial liabilities	145,600	110,200
Fair value | Corporate
Liabilities
Loans, net of unearned income	3,981	4,067
Fair value | Level 1
Assets
Investments	1,600	1,900
Securities borrowed and purchased under agreements to resell	0	0
Loans	0	0
Other financial assets	269,600	177,600
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	86,200	83,800
Securities borrowed and purchased under agreements to resell	106,900	98,100
Loans	0	4,700
Other financial assets	16,300	16,300
Liabilities
Deposits	978,400	875,500
Securities loaned and sold under agreements to repurchase	159,600	125,700
Long-term debt	185,000	187,300
Other financial liabilities	19,800	37,500
Fair value | Level 3
Assets
Investments	2,200	2,100
Securities borrowed and purchased under agreements to resell	0	0
Loans	711,200	673,000
Other financial assets	101,000	68,500
Liabilities
Deposits	203,900	191,200
Securities loaned and sold under agreements to repurchase	0	0
Long-term debt	29,800	16,500
Other financial liabilities	125,800	72,700
Fair value | Level 3 | Corporate
Fair value measurements additional disclosures
Unfunded lending commitments	10,400	5,100
Lease financing
Liabilities
Loans, net of unearned income	1,100	1,400
Lease financing | Corporate
Liabilities
Loans, net of unearned income	$ 1,060	$ 1,385