ALLOWANCE FOR CREDIT LOSSES ON LOANS AND LEASES - Changes in Allowance for Credit Losses (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Jan. 01, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	$ 196	$ 365	$ 188	$ 196	$ 180
Charge- Offs		(23)	(9)	(46)	(32)
Recoveries		4	3	13	9
Net Charge- Offs		(19)	(6)	(33)	(23)
Provision for Credit Losses		27	12	105	37
ASC 326 Adoption Impact	55
Initial ACL on PCD Loans				50
Balance at End of Period	196	373	194	373	194
Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			183		173
Charge- Offs			(7)		(23)
Recoveries			2		7
Net Charge- Offs			(5)		(16)
Provision for Credit Losses			11		32
Balance at End of Period			189		189
Acquired Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			5		7
Charge- Offs			(2)		(9)
Recoveries			1		2
Net Charge- Offs			(1)		(7)
Provision for Credit Losses			1		5
Balance at End of Period			5		5
Commercial Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	60	163		60
Charge- Offs		(3)		(8)
Recoveries		1		6
Net Charge- Offs		(2)		(2)
Provision for Credit Losses		23		48
ASC 326 Adoption Impact	38
Initial ACL on PCD Loans				40
Balance at End of Period	60	184		184
Commercial Real Estate [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			61		55
Charge- Offs			(1)		(3)
Recoveries			0		1
Net Charge- Offs			(1)		(2)
Provision for Credit Losses			(2)		5
Balance at End of Period			58		58
Commercial and Industrial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	53	98		53
Charge- Offs		(17)		(25)
Recoveries		1		3
Net Charge- Offs		(16)		(22)
Provision for Credit Losses		11		50
ASC 326 Adoption Impact	8
Initial ACL on PCD Loans				4
Balance at End of Period	53	93		93
Commercial and Industrial [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			52		49
Charge- Offs			(3)		(7)
Recoveries			1		3
Net Charge- Offs			(2)		(4)
Provision for Credit Losses			7		12
Balance at End of Period			57		57
Commercial Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	11	17		11
Charge- Offs		0		0
Recoveries		0		0
Net Charge- Offs		0		0
Provision for Credit Losses		0		6
ASC 326 Adoption Impact	0
Initial ACL on PCD Loans				0
Balance at End of Period	11	17		17
Commercial Leases [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			9		8
Charge- Offs			0		0
Recoveries			0		0
Net Charge- Offs			0		0
Provision for Credit Losses			1		2
Balance at End of Period			10		10
Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	9	1		9
Charge- Offs		(1)		(3)
Recoveries		0		0
Net Charge- Offs		(1)		(3)
Provision for Credit Losses		1		4
ASC 326 Adoption Impact	(9)
Initial ACL on PCD Loans				0
Balance at End of Period	9	1		1
Other [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			1		2
Charge- Offs			0		(2)
Recoveries			0		0
Net Charge- Offs			0		(2)
Provision for Credit Losses			1		2
Balance at End of Period			2		2
Total Commercial Loans and Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	133	279		133
Charge- Offs		(21)		(36)
Recoveries		2		9
Net Charge- Offs		(19)		(27)
Provision for Credit Losses		35		108
ASC 326 Adoption Impact	37
Initial ACL on PCD Loans				44
Balance at End of Period		295		295
Total Commercial Loans and Leases [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			123		114
Charge- Offs			(4)		(12)
Recoveries			1		4
Net Charge- Offs			(3)		(8)
Provision for Credit Losses			7		21
Balance at End of Period			127		127
Direct Installment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	13	25		13
Charge- Offs		0		(1)
Recoveries		0		0
Net Charge- Offs		0		(1)
Provision for Credit Losses		(1)		1
ASC 326 Adoption Impact	10
Initial ACL on PCD Loans				1
Balance at End of Period	13	24		24
Direct Installment [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			13		14
Charge- Offs			0		(1)
Recoveries			0		0
Net Charge- Offs			0		(1)
Provision for Credit Losses			(1)		(1)
Balance at End of Period			12		12
Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	22	33		22
Charge- Offs		(1)		(1)
Recoveries		1		1
Net Charge- Offs		0		0
Provision for Credit Losses		(2)		(1)
ASC 326 Adoption Impact	6
Initial ACL on PCD Loans				4
Balance at End of Period	22	31		31
Residential Mortgages [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			20		20
Charge- Offs			0		(1)
Recoveries			0		0
Net Charge- Offs			0		(1)
Provision for Credit Losses			3		4
Balance at End of Period			23		23
Indirect Installment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	19	17		19
Charge- Offs		(1)		(6)
Recoveries		1		3
Net Charge- Offs		0		(3)
Provision for Credit Losses		(5)		(6)
ASC 326 Adoption Impact	2
Initial ACL on PCD Loans				0
Balance at End of Period	19	12		12
Indirect Installment [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			18		15
Charge- Offs			(3)		(8)
Recoveries			1		3
Net Charge- Offs			(2)		(5)
Provision for Credit Losses			2		8
Balance at End of Period			18		18
Consumer Lines of Credit [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	9	11		9
Charge- Offs		0		(2)
Recoveries		0		0
Net Charge- Offs		0		(2)
Provision for Credit Losses		0		3
ASC 326 Adoption Impact	0
Initial ACL on PCD Loans				1
Balance at End of Period	9	11		11
Consumer Lines of Credit [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			9		10
Charge- Offs			0		(1)
Recoveries			0		0
Net Charge- Offs			0		(1)
Provision for Credit Losses			0		0
Balance at End of Period			9		9
Consumer Loan [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	63	86		63
Charge- Offs		(2)		(10)
Recoveries		2		4
Net Charge- Offs		0		(6)
Provision for Credit Losses		(8)		(3)
ASC 326 Adoption Impact	$ 18
Initial ACL on PCD Loans				6
Balance at End of Period		$ 78		$ 78
Consumer Loan [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			60		59
Charge- Offs			(3)		(11)
Recoveries			1		3
Net Charge- Offs			(2)		(8)
Provision for Credit Losses			4		11
Balance at End of Period			62		62
Purchased Credit Impaired Loans [Member] | Acquired Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			1		1
Charge- Offs			0		0
Recoveries			0		0
Net Charge- Offs			0		0
Provision for Credit Losses			0		0
Balance at End of Period			1		1
Other Acquired Loans [Member] | Acquired Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			4		6
Charge- Offs			(2)		(9)
Recoveries			1		2
Net Charge- Offs			(1)		(7)
Provision for Credit Losses			1		5
Balance at End of Period			$ 4		$ 4