FAIR VALUE MEASUREMENT - Estimate Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Assets
Loans	$ 5,799		$ 4,085
Liabilities
Deposits	1,233,660		1,070,590
Purchased credit-deteriorated	26,420	$ 20,841	12,783	$ 12,466	$ 12,329	$ 12,315
Loans, net of unearned income	685,292		699,483
Corporate
Assets
Loans	5,783		4,067
Liabilities
Purchased credit-deteriorated	6,824	$ 3,451	2,886	$ 2,787	$ 2,731	$ 2,811
Loans, net of unearned income	404,179		389,935
Carrying value
Assets
Investments	89,100		86,400
Securities borrowed and purchased under agreements to resell	108,400		98,100
Loans	652,100		681,200
Other Assets, Fair Value Disclosure	389,700		262,400
Liabilities
Deposits	1,231,200		1,068,300
Securities loaned and sold under agreements to repurchase	156,300		125,700
Long-term debt	217,800		193,000
Other Liabilities, Fair Value Disclosure	113,900		110,200
Purchased credit-deteriorated	26,400		12,800
Fair value
Assets
Investments	91,800		87,800
Securities borrowed and purchased under agreements to resell	108,400		98,100
Loans	677,500		677,700
Other Assets, Fair Value Disclosure	389,700		262,400
Liabilities
Deposits	1,231,500		1,066,700
Securities loaned and sold under agreements to repurchase	156,300		125,700
Long-term debt	223,900		203,800
Other Liabilities, Fair Value Disclosure	113,900		110,200
Fair value | Corporate
Liabilities
Loans, net of unearned income	5,783		4,067
Fair value | Level 1
Assets
Investments	1,200		1,900
Securities borrowed and purchased under agreements to resell	0		0
Loans	0		0
Other Assets, Fair Value Disclosure	294,400		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	88,100		83,800
Securities borrowed and purchased under agreements to resell	107,600		98,100
Loans	3,000		4,700
Other Assets, Fair Value Disclosure	15,400		16,300
Liabilities
Deposits	1,028,100		875,500
Securities loaned and sold under agreements to repurchase	156,300		125,700
Long-term debt	197,500		187,300
Other Liabilities, Fair Value Disclosure	19,100		37,500
Fair value | Level 3
Assets
Investments	2,500		2,100
Securities borrowed and purchased under agreements to resell	800		0
Loans	674,500		673,000
Other Assets, Fair Value Disclosure	79,900		68,500
Liabilities
Deposits	203,400		191,200
Securities loaned and sold under agreements to repurchase	0		0
Long-term debt	26,400		16,500
Other Liabilities, Fair Value Disclosure	94,800		72,700
Fair value | Level 3 | Corporate
Fair value measurements additional disclosures
Unfunded lending commitments	14,400		5,100
Lease financing
Liabilities
Loans, net of unearned income	900		1,400
Lease financing | Corporate
Liabilities
Loans, net of unearned income	$ 935		$ 1,385