Allowance for Credit Losses on Loans and Leases - Changes in Allowance for Credit Losses (Detail) - USD ($) $ in Millions		12 Months Ended
	Jan. 01, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	$ 196	$ 196	$ 180	$ 175
Charge- Offs			(42)	(70)
Recoveries			14	14
Net Charge- Offs			(28)	(56)
Provision for Credit Losses			44	61
Balance at End of Period	196	363	196	180
Unfunded Loan Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	3	3
Charge- Offs		0
Recoveries		0
Net Charge- Offs		0
Provision for Credit Losses		1
ASC 326 Adoption Impact	10
Initial ACL on PCD Loans		0
Balance at End of Period		14	3
Originated Loans And Leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	189	189	173	168
Charge- Offs			(33)	(63)
Recoveries			12	11
Net Charge- Offs			(21)	(52)
Provision for Credit Losses			37	57
Balance at End of Period			189	173
Allowance for credit losses on loans and leases and allowance for unfunded loan commitments
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	199	199
Charge- Offs		(81)
Recoveries		21
Net Charge- Offs		(60)
Provision for Credit Losses		123
ASC 326 Adoption Impact	65
Initial ACL on PCD Loans		50
Balance at End of Period		377	199
Allowance for credit losses on loans and leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	196	196
Charge- Offs		(81)
Recoveries		21
Net Charge- Offs		(60)
Provision for Credit Losses		122
ASC 326 Adoption Impact	55
Initial ACL on PCD Loans		50
Balance at End of Period		363	196
Total commercial loans and leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	133	133
Charge- Offs		(68)
Recoveries		15
Net Charge- Offs		(53)
Provision for Credit Losses		119
ASC 326 Adoption Impact	37
Initial ACL on PCD Loans		44
Balance at End of Period		280	133
Total commercial loans and leases | Unfunded Loan Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for Credit Losses		1
ASC 326 Adoption Impact		8
Total commercial loans and leases | Originated Loans And Leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	126	126	114	109
Charge- Offs			(17)	(34)
Recoveries			8	5
Net Charge- Offs			(9)	(29)
Provision for Credit Losses			21	34
Balance at End of Period			126	114
Commercial real estate
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	60	60
Charge- Offs		(31)
Recoveries		7
Net Charge- Offs		(24)
Provision for Credit Losses		67
ASC 326 Adoption Impact	38
Initial ACL on PCD Loans		40
Balance at End of Period	60	181	60
Commercial real estate | Originated Loans And Leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	60	60	55	50
Charge- Offs			(4)	(7)
Recoveries			4	3
Net Charge- Offs			0	(4)
Provision for Credit Losses			5	9
Balance at End of Period			60	55
Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	53	53
Charge- Offs		(32)
Recoveries		7
Net Charge- Offs		(25)
Provision for Credit Losses		41
ASC 326 Adoption Impact	8
Initial ACL on PCD Loans		4
Balance at End of Period	53	81	53
Commercial and industrial | Originated Loans And Leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	53	53	49	52
Charge- Offs			(10)	(20)
Recoveries			4	2
Net Charge- Offs			(6)	(18)
Provision for Credit Losses			10	15
Balance at End of Period			53	49
Commercial leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	11	11
Charge- Offs		(1)
Recoveries		0
Net Charge- Offs		(1)
Provision for Credit Losses		7
ASC 326 Adoption Impact	0
Initial ACL on PCD Loans		0
Balance at End of Period	11	17	11
Commercial leases | Originated Loans And Leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	11	11	8	5
Charge- Offs			0	(3)
Recoveries			0	0
Net Charge- Offs			0	(3)
Provision for Credit Losses			3	6
Balance at End of Period			11	8
Other
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	9	9
Charge- Offs		(4)
Recoveries		1
Net Charge- Offs		(3)
Provision for Credit Losses		4
ASC 326 Adoption Impact	(9)
Initial ACL on PCD Loans		0
Balance at End of Period	9	1	9
Other | Originated Loans And Leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	2	2	2	2
Charge- Offs			(3)	(4)
Recoveries			0	0
Net Charge- Offs			(3)	(4)
Provision for Credit Losses			3	4
Balance at End of Period			2	2
Total consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	63	63
Charge- Offs		(13)
Recoveries		6
Net Charge- Offs		(7)
Provision for Credit Losses		3
ASC 326 Adoption Impact	18
Initial ACL on PCD Loans		6
Balance at End of Period		83	63
Total consumer loans | Originated Loans And Leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	63	63	59	59
Charge- Offs			(16)	(29)
Recoveries			4	6
Net Charge- Offs			(12)	(23)
Provision for Credit Losses			16	23
Balance at End of Period			63	59
Direct installment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	13	13
Charge- Offs		(1)
Recoveries		1
Net Charge- Offs		0
Provision for Credit Losses		2
ASC 326 Adoption Impact	10
Initial ACL on PCD Loans		1
Balance at End of Period	13	26	13
Direct installment | Originated Loans And Leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	13	13	14	21
Charge- Offs			(1)	(17)
Recoveries			0	2
Net Charge- Offs			(1)	(15)
Provision for Credit Losses			0	8
Balance at End of Period			13	14
Residential mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	22	22
Charge- Offs		(2)
Recoveries		1
Net Charge- Offs		(1)
Provision for Credit Losses		3
ASC 326 Adoption Impact	6
Initial ACL on PCD Loans		4
Balance at End of Period	22	34	22
Residential mortgages | Originated Loans And Leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	22	22	20	16
Charge- Offs			(2)	0
Recoveries			0	0
Net Charge- Offs			(2)	0
Provision for Credit Losses			4	4
Balance at End of Period			22	20
Indirect installment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	19	19
Charge- Offs		(8)
Recoveries		4
Net Charge- Offs		(4)
Provision for Credit Losses		(6)
ASC 326 Adoption Impact	2
Initial ACL on PCD Loans		0
Balance at End of Period	19	11	19
Indirect installment | Originated Loans And Leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	19	19	15	12
Charge- Offs			(11)	(9)
Recoveries			4	4
Net Charge- Offs			(7)	(5)
Provision for Credit Losses			11	8
Balance at End of Period			19	15
Consumer lines of credit
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	9	9
Charge- Offs		(2)
Recoveries		0
Net Charge- Offs		(2)
Provision for Credit Losses		4
ASC 326 Adoption Impact	0
Initial ACL on PCD Loans		1
Balance at End of Period	9	12	9
Consumer lines of credit | Unfunded Loan Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for Credit Losses		0
ASC 326 Adoption Impact		2
Consumer lines of credit | Originated Loans And Leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	9	9	10	10
Charge- Offs			(2)	(3)
Recoveries			0	0
Net Charge- Offs			(2)	(3)
Provision for Credit Losses			1	3
Balance at End of Period			9	10
Loans acquired in a business combination | Acquired Loans
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	$ 7	$ 7	7	7
Charge- Offs			(9)	(7)
Recoveries			2	3
Net Charge- Offs			(7)	(4)
Provision for Credit Losses			7	4
Balance at End of Period			$ 7	$ 7