FAIR VALUE MEASUREMENT - Estimate Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Assets
Loans	$ 5,524		$ 4,085
Liabilities
Deposits	1,262,623		1,070,590
Purchased credit-deteriorated	26,426	$ 26,420	12,783	$ 12,530	$ 12,466	$ 12,315
Loans, net of unearned income	666,911		699,483
Corporate
Assets
Loans	5,510		4,067
Liabilities
Purchased credit-deteriorated	6,938	$ 6,824	2,886	$ 2,803	$ 2,787	$ 2,811
Loans, net of unearned income	386,886		389,935
Carrying value
Assets
Investments	101,800		86,400
Securities borrowed and purchased under agreements to resell	113,700		98,100
Loans	634,200		681,200
Other Assets, Fair Value Disclosure	403,000		262,400
Liabilities
Deposits	1,260,100		1,068,300
Securities loaned and sold under agreements to repurchase	146,300		125,700
Long-term debt	210,200		193,000
Other Liabilities, Fair Value Disclosure	112,900		110,200
Purchased credit-deteriorated	26,400		12,800
Fair value
Assets
Investments	104,800		87,800
Securities borrowed and purchased under agreements to resell	113,700		98,100
Loans	658,300		677,700
Other Assets, Fair Value Disclosure	403,000		262,400
Liabilities
Deposits	1,260,000		1,066,700
Securities loaned and sold under agreements to repurchase	146,300		125,700
Long-term debt	223,800		203,800
Other Liabilities, Fair Value Disclosure	112,900		110,200
Fair value | Corporate
Liabilities
Loans, net of unearned income	5,510		4,067
Fair value | Level 1
Assets
Investments	16,700		1,900
Securities borrowed and purchased under agreements to resell	0		0
Loans	0		0
Other Assets, Fair Value Disclosure	305,600		177,600
Liabilities
Deposits	0		0
Securities loaned and sold under agreements to repurchase	0		0
Long-term debt	0		0
Other Liabilities, Fair Value Disclosure	0		0
Fair value | Level 2
Assets
Investments	85,300		83,800
Securities borrowed and purchased under agreements to resell	113,700		98,100
Loans	500		4,700
Other Assets, Fair Value Disclosure	18,100		16,300
Liabilities
Deposits	1,067,400		875,500
Securities loaned and sold under agreements to repurchase	146,300		125,700
Long-term debt	195,700		187,300
Other Liabilities, Fair Value Disclosure	20,400		37,500
Fair value | Level 3
Assets
Investments	2,800		2,100
Securities borrowed and purchased under agreements to resell	0		0
Loans	657,800		673,000
Other Assets, Fair Value Disclosure	79,300		68,500
Liabilities
Deposits	192,600		191,200
Securities loaned and sold under agreements to repurchase	0		0
Long-term debt	28,100		16,500
Other Liabilities, Fair Value Disclosure	92,500		72,700
Fair value | Level 3 | Corporate
Fair value measurements additional disclosures
Unfunded lending commitments	9,400		5,100
Lease financing
Liabilities
Loans, net of unearned income	800		1,400
Lease financing | Corporate
Liabilities
Loans, net of unearned income	$ 767		$ 1,385