Allowance for Credit Losses - Summary of Changes in Allowance for Credit Losses by Loan and Lease Class (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				$ 180				$ 175	$ 180	$ 175	$ 158
Charge- Offs									(42)	(70)	(60)
Recoveries									14	14	16
Net Charge- Offs									(28)	(56)	(44)
Provision for credit losses	$ 7	$ 12	$ 11	14	$ 15	$ 16	$ 16	14	44	61	61
Balance at End of Year	196				180				196	180	175
Originated Loans and Leases
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				173				168	173	168	151
Charge- Offs									(33)	(63)	(58)
Recoveries									12	11	11
Net Charge- Offs									(21)	(52)	(47)
Provision for credit losses									37	57	64
Balance at End of Year	189				173				189	173	168
Loans Acquired in a Business Combination
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				7				7	7	7	7
Charge- Offs									(9)	(7)	(2)
Recoveries									2	3	5
Net Charge- Offs									(7)	(4)	3
Provision for credit losses									7	4	(3)
Balance at End of Year	7				7				7	7	7
Total commercial loans and leases | Originated Loans and Leases
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				114				109	114	109	99
Charge- Offs									(17)	(34)	(34)
Recoveries									8	5	5
Net Charge- Offs									(9)	(29)	(29)
Provision for credit losses									21	34	39
Balance at End of Year	126				114				126	114	109
Total commercial loans and leases | Loans Acquired in a Business Combination
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				6					6
Balance at End of Year	4				6				4	6
Commercial real estate | Originated Loans and Leases
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				55				50	55	50	47
Charge- Offs									(4)	(7)	(2)
Recoveries									4	3	2
Net Charge- Offs									0	(4)	0
Provision for credit losses									5	9	3
Balance at End of Year	60				55				60	55	50
Commercial real estate | Loans Acquired in a Business Combination
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				2					2
Balance at End of Year	4				2				4	2
Commercial and industrial | Originated Loans and Leases
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				49				52	49	52	48
Charge- Offs									(10)	(20)	(27)
Recoveries									4	2	2
Net Charge- Offs									(6)	(18)	(25)
Provision for credit losses									10	15	29
Balance at End of Year	53				49				53	49	52
Commercial and industrial | Loans Acquired in a Business Combination
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				4					4
Balance at End of Year	0				4				0	4
Commercial leases | Originated Loans and Leases
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				8				5	8	5	3
Charge- Offs									0	(3)	(1)
Recoveries									0	0	0
Net Charge- Offs									0	(3)	(1)
Provision for credit losses									3	6	3
Balance at End of Year	11				8				11	8	5
Other | Originated Loans and Leases
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				2				2	2	2	1
Charge- Offs									(3)	(4)	(4)
Recoveries									0	0	1
Net Charge- Offs									(3)	(4)	(3)
Provision for credit losses									3	4	4
Balance at End of Year	2				2				2	2	2
Total consumer loans | Originated Loans and Leases
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				59				59	59	59	52
Charge- Offs									(16)	(29)	(24)
Recoveries									4	6	6
Net Charge- Offs									(12)	(23)	(18)
Provision for credit losses									16	23	25
Balance at End of Year	63				59				63	59	59
Total consumer loans | Loans Acquired in a Business Combination
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				1					1
Balance at End of Year	3				1				3	1
Direct installment | Originated Loans and Leases
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				14				21	14	21	21
Charge- Offs									(1)	(17)	(12)
Recoveries									0	2	2
Net Charge- Offs									(1)	(15)	(10)
Provision for credit losses									0	8	10
Balance at End of Year	13				14				13	14	21
Direct installment | Loans Acquired in a Business Combination
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				1					1
Balance at End of Year	1				1				1	1
Residential mortgages | Originated Loans and Leases
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				20				16	20	16	10
Charge- Offs									(2)	0	0
Recoveries									0	0	0
Net Charge- Offs									(2)	0	0
Provision for credit losses									4	4	6
Balance at End of Year	22				20				22	20	16
Residential mortgages | Loans Acquired in a Business Combination
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				0					0
Balance at End of Year	2				0				2	0
Indirect installment | Originated Loans and Leases
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				15				12	15	12	11
Charge- Offs									(11)	(9)	(10)
Recoveries									4	4	4
Net Charge- Offs									(7)	(5)	(6)
Provision for credit losses									11	8	7
Balance at End of Year	19				15				19	15	12
Consumer lines of credit | Originated Loans and Leases
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				10				10	10	10	10
Charge- Offs									(2)	(3)	(2)
Recoveries									0	0	0
Net Charge- Offs									(2)	(3)	(2)
Provision for credit losses									1	3	2
Balance at End of Year	9				10				9	10	10
Purchased credit-impaired loans | Loans Acquired in a Business Combination
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				1				1	1	1	1
Charge- Offs									0	0	(1)
Recoveries									0	0	0
Net Charge- Offs									0	0	(1)
Provision for credit losses									0	0	1
Balance at End of Year	1				1				1	1	1
Other loans acquired in a business combination | Loans Acquired in a Business Combination
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year				$ 6				$ 6	6	6	6
Charge- Offs									(9)	(7)	(1)
Recoveries									2	3	5
Net Charge- Offs									(7)	(4)	4
Provision for credit losses									7	4	(4)
Balance at End of Year	$ 6				$ 6				$ 6	$ 6	$ 6