ALLOWANCE FOR CREDIT LOSSES - Changes in Allowance for Credit Losses (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	$ 196	$ 180
Charge- Offs	(12)	(10)
Recoveries	6	2
Net Charge- Offs	(6)	(8)
Provision for Credit Losses	48	14
ASC 326 Adoption Impact	55
Initial ACL on PCD Loans	50
Balance at End of Period	343	186
Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		173
Charge- Offs		(7)
Recoveries		2
Net Charge- Offs		(5)
Provision for Credit Losses		9
Balance at End of Period		177
Acquired Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		7
Charge- Offs		(3)
Recoveries		0
Net Charge- Offs		(3)
Provision for Credit Losses		5
Balance at End of Period		9
Commercial Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	60
Charge- Offs	(2)
Recoveries	4
Net Charge- Offs	2
Provision for Credit Losses	12
ASC 326 Adoption Impact	38
Initial ACL on PCD Loans	40
Balance at End of Period	152
Commercial Real Estate [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		55
Charge- Offs		(1)
Recoveries		0
Net Charge- Offs		(1)
Provision for Credit Losses		3
Balance at End of Period		57
Commercial and Industrial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	53
Charge- Offs	(4)
Recoveries	1
Net Charge- Offs	(3)
Provision for Credit Losses	26
ASC 326 Adoption Impact	8
Initial ACL on PCD Loans	4
Balance at End of Period	88
Commercial and Industrial [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		49
Charge- Offs		(1)
Recoveries		1
Net Charge- Offs		0
Provision for Credit Losses		3
Balance at End of Period		52
Commercial Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	11
Charge- Offs	0
Recoveries	0
Net Charge- Offs	0
Provision for Credit Losses	2
ASC 326 Adoption Impact	0
Initial ACL on PCD Loans	0
Balance at End of Period	13
Commercial Leases [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		8
Charge- Offs		0
Recoveries		0
Net Charge- Offs		0
Provision for Credit Losses		0
Balance at End of Period		8
Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	9
Charge- Offs	(1)
Recoveries	0
Net Charge- Offs	(1)
Provision for Credit Losses	2
ASC 326 Adoption Impact	(9)
Initial ACL on PCD Loans	0
Balance at End of Period	1
Other [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		2
Charge- Offs		(1)
Recoveries		0
Net Charge- Offs		(1)
Provision for Credit Losses		1
Balance at End of Period		2
Total Commercial Loans and Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	133
Charge- Offs	(7)
Recoveries	5
Net Charge- Offs	(2)
Provision for Credit Losses	42
ASC 326 Adoption Impact	37
Initial ACL on PCD Loans	44
Balance at End of Period	254
Total Commercial Loans and Leases [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		114
Charge- Offs		(3)
Recoveries		1
Net Charge- Offs		(2)
Provision for Credit Losses		7
Balance at End of Period		119
Direct Installment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	13
Charge- Offs	(1)
Recoveries	0
Net Charge- Offs	(1)
Provision for Credit Losses	3
ASC 326 Adoption Impact	10
Initial ACL on PCD Loans	1
Balance at End of Period	26
Direct Installment [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		14
Charge- Offs		(1)
Recoveries		0
Net Charge- Offs		(1)
Provision for Credit Losses		(1)
Balance at End of Period		12
Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	22
Charge- Offs	0
Recoveries	0
Net Charge- Offs	0
Provision for Credit Losses	(1)
ASC 326 Adoption Impact	6
Initial ACL on PCD Loans	4
Balance at End of Period	31
Residential Mortgages [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		20
Charge- Offs		0
Recoveries		0
Net Charge- Offs		0
Provision for Credit Losses		(1)
Balance at End of Period		19
Indirect Installment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	19
Charge- Offs	(3)
Recoveries	1
Net Charge- Offs	(2)
Provision for Credit Losses	2
ASC 326 Adoption Impact	2
Initial ACL on PCD Loans	0
Balance at End of Period	21
Indirect Installment [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		15
Charge- Offs		(3)
Recoveries		1
Net Charge- Offs		(2)
Provision for Credit Losses		4
Balance at End of Period		17
Consumer Lines of Credit [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	9
Charge- Offs	(1)
Recoveries	0
Net Charge- Offs	(1)
Provision for Credit Losses	2
ASC 326 Adoption Impact	0
Initial ACL on PCD Loans	1
Balance at End of Period	11
Consumer Lines of Credit [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		10
Charge- Offs		0
Recoveries		0
Net Charge- Offs		0
Provision for Credit Losses		0
Balance at End of Period		10
Consumer Loan [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	63
Charge- Offs	(5)
Recoveries	1
Net Charge- Offs	(4)
Provision for Credit Losses	6
ASC 326 Adoption Impact	18
Initial ACL on PCD Loans	6
Balance at End of Period	$ 89
Consumer Loan [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		59
Charge- Offs		(4)
Recoveries		1
Net Charge- Offs		(3)
Provision for Credit Losses		2
Balance at End of Period		58
Purchased Credit Impaired Loans [Member] | Acquired Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		1
Charge- Offs		0
Recoveries		0
Net Charge- Offs		0
Provision for Credit Losses		0
Balance at End of Period		1
Other Acquired Loans [Member] | Acquired Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		6
Charge- Offs		(3)
Recoveries		0
Net Charge- Offs		(3)
Provision for Credit Losses		5
Balance at End of Period		$ 8