ALLOWANCE FOR CREDIT LOSSES - Changes in Allowance for Credit Losses (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Jan. 01, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	$ 196.0	$ 343.0	$ 186.0	$ 196.0	$ 180.0
Charge- Offs		(11.0)	(13.0)	(23.0)	(23.0)
Recoveries		3.0	4.0	9.0	6.0
Net Charge- Offs		(8.5)	(9.0)	(14.2)	(16.6)
Provision for Credit Losses		30.0	11.0	78.0	25.0
ASC 326 Adoption Impact	55.0
Initial ACL on PCD Loans				50.0
Balance at End of Period	196.0	365.0	188.0	365.0	188.0
Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			177.0		173.0
Charge- Offs			(9.0)		(16.0)
Recoveries			3.0		5.0
Net Charge- Offs			(6.0)		(11.0)
Provision for Credit Losses			12.0		21.0
Balance at End of Period			183.0		183.0
Acquired Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			9.0		7.0
Charge- Offs			(4.0)		(7.0)
Recoveries			1.0		1.0
Net Charge- Offs			(3.0)		(6.0)
Provision for Credit Losses			(1.0)		4.0
Balance at End of Period			5.0		5.0
Commercial Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	60.0	152.0		60.0
Charge- Offs		(3.0)		(5.0)
Recoveries		1.0		5.0
Net Charge- Offs		(2.0)		0.0
Provision for Credit Losses		13.0		25.0
ASC 326 Adoption Impact	38.0
Initial ACL on PCD Loans				40.0
Balance at End of Period	60.0	163.0		163.0
Commercial Real Estate [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			57.0		55.0
Charge- Offs			(1.0)		(2.0)
Recoveries			1.0		1.0
Net Charge- Offs			0.0		(1.0)
Provision for Credit Losses			4.0		7.0
Balance at End of Period			61.0		61.0
Commercial and Industrial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	53.0	88.0		53.0
Charge- Offs		(4.0)		(8.0)
Recoveries		1.0		2.0
Net Charge- Offs		(3.0)		(6.0)
Provision for Credit Losses		13.0		39.0
ASC 326 Adoption Impact	8.0
Initial ACL on PCD Loans				4.0
Balance at End of Period	53.0	98.0		98.0
Commercial and Industrial [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			52.0		49.0
Charge- Offs			(3.0)		(4.0)
Recoveries			1.0		2.0
Net Charge- Offs			(2.0)		(2.0)
Provision for Credit Losses			2.0		5.0
Balance at End of Period			52.0		52.0
Commercial Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	11.0	13.0		11.0
Charge- Offs		0.0		0.0
Recoveries		0.0		0.0
Net Charge- Offs		0.0		0.0
Provision for Credit Losses		4.0		6.0
ASC 326 Adoption Impact	0.0
Initial ACL on PCD Loans				0.0
Balance at End of Period	11.0	17.0		17.0
Commercial Leases [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			8.0		8.0
Charge- Offs			0.0		0.0
Recoveries			0.0		0.0
Net Charge- Offs			0.0		0.0
Provision for Credit Losses			1.0		1.0
Balance at End of Period			9.0		9.0
Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	9.0	1.0		9.0
Charge- Offs		(1.0)		(2.0)
Recoveries		0.0
Net Charge- Offs		(1.0)		(2.0)
Provision for Credit Losses		1.0		3.0
ASC 326 Adoption Impact	(9.0)
Initial ACL on PCD Loans				0.0
Balance at End of Period	9.0	1.0		1.0
Other [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			2.0		2.0
Charge- Offs			(1.0)		(2.0)
Recoveries			0.0		0.0
Net Charge- Offs			(1.0)		(2.0)
Provision for Credit Losses			0.0		1.0
Balance at End of Period			1.0		1.0
Total Commercial Loans and Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	133.0	254.0		133.0
Charge- Offs		(8.0)		(15.0)
Recoveries		2.0		7.0
Net Charge- Offs		(6.0)		(8.0)
Provision for Credit Losses		31.0		73.0
ASC 326 Adoption Impact	37.0
Initial ACL on PCD Loans				44.0
Balance at End of Period		279.0		279.0
Total Commercial Loans and Leases [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			119.0		114.0
Charge- Offs			(5.0)		(8.0)
Recoveries			2.0		3.0
Net Charge- Offs			(3.0)		(5.0)
Provision for Credit Losses			7.0		14.0
Balance at End of Period			123.0		123.0
Direct Installment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	13.0	26.0		13.0
Charge- Offs		0.0		(1.0)
Recoveries		0.0		0.0
Net Charge- Offs		0.0		(1.0)
Provision for Credit Losses		(1.0)		2.0
ASC 326 Adoption Impact	10.0
Initial ACL on PCD Loans				1.0
Balance at End of Period	13.0	25.0		25.0
Direct Installment [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			12.0		14.0
Charge- Offs			0.0		(1.0)
Recoveries			0.0		0.0
Net Charge- Offs			0.0		(1.0)
Provision for Credit Losses			1.0		0.0
Balance at End of Period			13.0		13.0
Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	22.0	31.0		22.0
Charge- Offs		0.0		0.0
Recoveries		0.0		0.0
Net Charge- Offs		0.0		0.0
Provision for Credit Losses		2.0		1.0
ASC 326 Adoption Impact	6.0
Initial ACL on PCD Loans				4.0
Balance at End of Period	22.0	33.0		33.0
Residential Mortgages [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			19.0		20.0
Charge- Offs			(1.0)		(1.0)
Recoveries			0.0		0.0
Net Charge- Offs			(1.0)		(1.0)
Provision for Credit Losses			2.0		1.0
Balance at End of Period			20.0		20.0
Indirect Installment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	19.0	21.0		19.0
Charge- Offs		(2.0)		(5.0)
Recoveries		1.0		2.0
Net Charge- Offs		(1.0)		(3.0)
Provision for Credit Losses		(3.0)		(1.0)
ASC 326 Adoption Impact	2.0
Initial ACL on PCD Loans				0.0
Balance at End of Period	19.0	17.0		17.0
Indirect Installment [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			17.0		15.0
Charge- Offs			(2.0)		(5.0)
Recoveries			1.0		2.0
Net Charge- Offs			(1.0)		(3.0)
Provision for Credit Losses			2.0		6.0
Balance at End of Period			18.0		18.0
Consumer Lines of Credit [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	9.0	11.0		9.0
Charge- Offs		(1.0)		(2.0)
Recoveries		0.0		0.0
Net Charge- Offs		(1.0)		(2.0)
Provision for Credit Losses		1.0		3.0
ASC 326 Adoption Impact	0.0
Initial ACL on PCD Loans				1.0
Balance at End of Period	9.0	11.0		11.0
Consumer Lines of Credit [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			10.0		10.0
Charge- Offs			(1.0)		(1.0)
Recoveries			0.0		0.0
Net Charge- Offs			(1.0)		(1.0)
Provision for Credit Losses			0.0		0.0
Balance at End of Period			9.0		9.0
Consumer Loan [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	63.0	89.0		63.0
Charge- Offs		(3.0)		(8.0)
Recoveries		1.0		2.0
Net Charge- Offs		(2.0)		(6.0)
Provision for Credit Losses		(1.0)		5.0
ASC 326 Adoption Impact	$ 18.0
Initial ACL on PCD Loans				6.0
Balance at End of Period		$ 86.0		$ 86.0
Consumer Loan [Member] | Originated Loans And Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			58.0		59.0
Charge- Offs			(4.0)		(8.0)
Recoveries			1.0		2.0
Net Charge- Offs			(3.0)		(6.0)
Provision for Credit Losses			5.0		7.0
Balance at End of Period			60.0		60.0
Purchased Credit Impaired Loans [Member] | Acquired Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			1.0		1.0
Charge- Offs			0.0		0.0
Recoveries			0.0		0.0
Net Charge- Offs			0.0		0.0
Provision for Credit Losses			0.0		0.0
Balance at End of Period			1.0		1.0
Other Acquired Loans [Member] | Acquired Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			8.0		6.0
Charge- Offs			(4.0)		(7.0)
Recoveries			1.0		1.0
Net Charge- Offs			(3.0)		(6.0)
Provision for Credit Losses			(1.0)		4.0
Balance at End of Period			$ 4.0		$ 4.0