FAIR VALUE MEASUREMENTS - Schedule of Financial Instruments not Recorded at Fair Value (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Financial assets:
Securities held to maturity, carrying value	[1]	$ 3,202	$ 4,165
Securities held-to-maturity, fair value		3,242	4,041
Other investment securities, at cost, carrying value		807	834
Other investment securities, at cost, estimated fair value		807	834
Other loans held for sale, carrying value		1,384	101
Other loans held for sale, fair value		1,384	101
Loans and leases, carrying value		119,088	116,660
Loans and leases, fair value		119,792	116,627
Financial liabilities:
Deposits, carrying value		125,313	119,575
Deposits, fair value		125,340	119,503
Federal funds purchased and securities sold under agreements to repurchase, carrying value		265	1,156
Federal funds purchased and securities sold under agreements to repurchase, fair value		265	1,156
Other short-term borrowed funds, carrying value		9	161
Other short-term borrowed funds, fair value		9	161
Long-term borrowed funds, carrying value		14,047	15,925
Long-term borrowed funds, fair value		14,228	15,877
Level 1
Financial assets:
Securities held to maturity, carrying value		0	0
Securities held-to-maturity, fair value		0	0
Other investment securities, at cost, carrying value		0	0
Other investment securities, at cost, estimated fair value		0	0
Other loans held for sale, carrying value		0	0
Other loans held for sale, fair value		0	0
Loans and leases, carrying value		0	0
Loans and leases, fair value		0	0
Financial liabilities:
Deposits, carrying value		0	0
Deposits, fair value		0	0
Federal funds purchased and securities sold under agreements to repurchase, carrying value		0	0
Federal funds purchased and securities sold under agreements to repurchase, fair value		0	0
Other short-term borrowed funds, carrying value		0	0
Other short-term borrowed funds, fair value		0	0
Long-term borrowed funds, carrying value		0	0
Long-term borrowed funds, fair value		0	0
Level 2
Financial assets:
Securities held to maturity, carrying value		3,202	4,165
Securities held-to-maturity, fair value		3,242	4,041
Other investment securities, at cost, carrying value		807	834
Other investment securities, at cost, estimated fair value		807	834
Other loans held for sale, carrying value		0	0
Other loans held for sale, fair value		0	0
Loans and leases, carrying value		312	338
Loans and leases, fair value		312	338
Financial liabilities:
Deposits, carrying value		125,313	119,575
Deposits, fair value		125,340	119,503
Federal funds purchased and securities sold under agreements to repurchase, carrying value		265	1,156
Federal funds purchased and securities sold under agreements to repurchase, fair value		265	1,156
Other short-term borrowed funds, carrying value		9	161
Other short-term borrowed funds, fair value		9	161
Long-term borrowed funds, carrying value		14,047	15,925
Long-term borrowed funds, fair value		14,228	15,877
Level 3
Financial assets:
Securities held to maturity, carrying value		0	0
Securities held-to-maturity, fair value		0	0
Other investment securities, at cost, carrying value		0	0
Other investment securities, at cost, estimated fair value		0	0
Other loans held for sale, carrying value		1,384	101
Other loans held for sale, fair value		1,384	101
Loans and leases, carrying value		118,776	116,322
Loans and leases, fair value		119,480	116,289
Financial liabilities:
Deposits, carrying value		0	0
Deposits, fair value		0	0
Federal funds purchased and securities sold under agreements to repurchase, carrying value		0	0
Federal funds purchased and securities sold under agreements to repurchase, fair value		0	0
Other short-term borrowed funds, carrying value		0	0
Other short-term borrowed funds, fair value		0	0
Long-term borrowed funds, carrying value		0	0
Long-term borrowed funds, fair value		$ 0	$ 0

[1] Includes only collateral pledged by the Company where counterparties have the right to sell or pledge the collateral.