Loans and Leases - Age Analysis of Past Due Loans, by Class (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	$ 23,289	$ 22,153
Total commercial loans and leases
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	14,721	13,761
Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	8,960	8,786
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	5,308	4,556
Commercial leases
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	432	373
Other
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	21	46
Total consumer loans
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	8,568	8,392
Direct installment
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	1,821	1,764
Residential mortgages
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	3,374	3,113
Indirect installment
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	1,922	1,933
Consumer lines of credit
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	1,451	1,582
Originated Loans and Leases
Financing Receivable, Past Due [Line Items]
Total Past Due	145	116
Non- Accrual	77	58
Current	20,252	17,946
Total Loans and Leases	20,397	18,062
Originated Loans and Leases | Total commercial loans and leases
Financing Receivable, Past Due [Line Items]
Total Past Due	80	52
Non- Accrual	56	39
Current	12,550	10,678
Total Loans and Leases	12,630	10,730
Originated Loans and Leases | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Past Due	36	24
Non- Accrual	26	17
Current	7,078	6,147
Total Loans and Leases	7,114	6,171
Originated Loans and Leases | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total Past Due	37	24
Non- Accrual	28	19
Current	5,026	4,116
Total Loans and Leases	5,063	4,140
Originated Loans and Leases | Commercial leases
Financing Receivable, Past Due [Line Items]
Total Past Due	6	3
Non- Accrual	1	2
Current	426	370
Total Loans and Leases	432	373
Originated Loans and Leases | Other
Financing Receivable, Past Due [Line Items]
Total Past Due	1	1
Non- Accrual	1	1
Current	20	45
Total Loans and Leases	21	46
Originated Loans and Leases | Total consumer loans
Financing Receivable, Past Due [Line Items]
Total Past Due	65	64
Non- Accrual	21	19
Current	7,702	7,268
Total Loans and Leases	7,767	7,332
Originated Loans and Leases | Direct installment
Financing Receivable, Past Due [Line Items]
Total Past Due	15	16
Non- Accrual	7	8
Current	1,743	1,652
Total Loans and Leases	1,758	1,668
Originated Loans and Leases | Residential mortgages
Financing Receivable, Past Due [Line Items]
Total Past Due	22	25
Non- Accrual	8	6
Current	2,973	2,587
Total Loans and Leases	2,995	2,612
Originated Loans and Leases | Indirect installment
Financing Receivable, Past Due [Line Items]
Total Past Due	19	14
Non- Accrual	3	2
Current	1,903	1,919
Total Loans and Leases	1,922	1,933
Originated Loans and Leases | Consumer lines of credit
Financing Receivable, Past Due [Line Items]
Total Past Due	9	9
Non- Accrual	3	3
Current	1,083	1,110
Total Loans and Leases	1,092	1,119
Loans Acquired in a Business Combination
Financing Receivable, Past Due [Line Items]
Total Past Due	73	120
Non- Accrual	4	21
Current	3,000	4,193
(Discount)/ Premium	(181)	(222)
Total Loans and Leases	2,892	4,091
Loans Acquired in a Business Combination | Total commercial loans and leases
Financing Receivable, Past Due [Line Items]
Total Past Due	48	84
Non- Accrual	3	20
Current	2,201	3,143
(Discount)/ Premium	(158)	(196)
Total Loans and Leases	2,091	3,031
Loans Acquired in a Business Combination | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Past Due	43	60
Non- Accrual	3	3
Current	1,942	2,723
(Discount)/ Premium	(139)	(168)
Total Loans and Leases	1,846	2,615
Loans Acquired in a Business Combination | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total Past Due	5	24
Non- Accrual	0	17
Current	259	420
(Discount)/ Premium	(19)	(28)
Total Loans and Leases	245	416
Loans Acquired in a Business Combination | Total consumer loans
Financing Receivable, Past Due [Line Items]
Total Past Due	25	36
Non- Accrual	1	1
Current	799	1,050
(Discount)/ Premium	(23)	(26)
Total Loans and Leases	801	1,060
Loans Acquired in a Business Combination | Direct installment
Financing Receivable, Past Due [Line Items]
Total Past Due	3	5
Non- Accrual	0	0
Current	60	91
(Discount)/ Premium	0	0
Total Loans and Leases	63	96
Loans Acquired in a Business Combination | Residential mortgages
Financing Receivable, Past Due [Line Items]
Total Past Due	12	19
Non- Accrual	0	0
Current	382	498
(Discount)/ Premium	(15)	(16)
Total Loans and Leases	379	501
Loans Acquired in a Business Combination | Consumer lines of credit
Financing Receivable, Past Due [Line Items]
Total Past Due	10	12
Non- Accrual	1	1
Current	357	461
(Discount)/ Premium	(8)	(10)
Total Loans and Leases	359	463
30-89 Days Past Due | Originated Loans and Leases
Financing Receivable, Past Due [Line Items]
Total Past Due	63	53
30-89 Days Past Due | Originated Loans and Leases | Total commercial loans and leases
Financing Receivable, Past Due [Line Items]
Total Past Due	24	13
30-89 Days Past Due | Originated Loans and Leases | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Past Due	10	7
30-89 Days Past Due | Originated Loans and Leases | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total Past Due	9	5
30-89 Days Past Due | Originated Loans and Leases | Commercial leases
Financing Receivable, Past Due [Line Items]
Total Past Due	5	1
30-89 Days Past Due | Originated Loans and Leases | Other
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
30-89 Days Past Due | Originated Loans and Leases | Total consumer loans
Financing Receivable, Past Due [Line Items]
Total Past Due	39	40
30-89 Days Past Due | Originated Loans and Leases | Direct installment
Financing Receivable, Past Due [Line Items]
Total Past Due	7	8
30-89 Days Past Due | Originated Loans and Leases | Residential mortgages
Financing Receivable, Past Due [Line Items]
Total Past Due	12	16
30-89 Days Past Due | Originated Loans and Leases | Indirect installment
Financing Receivable, Past Due [Line Items]
Total Past Due	15	11
30-89 Days Past Due | Originated Loans and Leases | Consumer lines of credit
Financing Receivable, Past Due [Line Items]
Total Past Due	5	5
30-89 Days Past Due | Loans Acquired in a Business Combination
Financing Receivable, Past Due [Line Items]
Total Past Due	32	46
30-89 Days Past Due | Loans Acquired in a Business Combination | Total commercial loans and leases
Financing Receivable, Past Due [Line Items]
Total Past Due	14	22
30-89 Days Past Due | Loans Acquired in a Business Combination | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Past Due	12	19
30-89 Days Past Due | Loans Acquired in a Business Combination | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total Past Due	2	3
30-89 Days Past Due | Loans Acquired in a Business Combination | Total consumer loans
Financing Receivable, Past Due [Line Items]
Total Past Due	18	24
30-89 Days Past Due | Loans Acquired in a Business Combination | Direct installment
Financing Receivable, Past Due [Line Items]
Total Past Due	3	3
30-89 Days Past Due | Loans Acquired in a Business Combination | Residential mortgages
Financing Receivable, Past Due [Line Items]
Total Past Due	8	13
30-89 Days Past Due | Loans Acquired in a Business Combination | Consumer lines of credit
Financing Receivable, Past Due [Line Items]
Total Past Due	7	8
≥ 90 Days Past Due and Still Accruing | Originated Loans and Leases
Financing Receivable, Past Due [Line Items]
Total Past Due	5	5
≥ 90 Days Past Due and Still Accruing | Originated Loans and Leases | Total commercial loans and leases
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
≥ 90 Days Past Due and Still Accruing | Originated Loans and Leases | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
≥ 90 Days Past Due and Still Accruing | Originated Loans and Leases | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
≥ 90 Days Past Due and Still Accruing | Originated Loans and Leases | Commercial leases
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
≥ 90 Days Past Due and Still Accruing | Originated Loans and Leases | Other
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
≥ 90 Days Past Due and Still Accruing | Originated Loans and Leases | Total consumer loans
Financing Receivable, Past Due [Line Items]
Total Past Due	5	5
≥ 90 Days Past Due and Still Accruing | Originated Loans and Leases | Direct installment
Financing Receivable, Past Due [Line Items]
Total Past Due	1	0
≥ 90 Days Past Due and Still Accruing | Originated Loans and Leases | Residential mortgages
Financing Receivable, Past Due [Line Items]
Total Past Due	2	3
≥ 90 Days Past Due and Still Accruing | Originated Loans and Leases | Indirect installment
Financing Receivable, Past Due [Line Items]
Total Past Due	1	1
≥ 90 Days Past Due and Still Accruing | Originated Loans and Leases | Consumer lines of credit
Financing Receivable, Past Due [Line Items]
Total Past Due	1	1
≥ 90 Days Past Due and Still Accruing | Loans Acquired in a Business Combination
Financing Receivable, Past Due [Line Items]
Total Past Due	37	53
≥ 90 Days Past Due and Still Accruing | Loans Acquired in a Business Combination | Total commercial loans and leases
Financing Receivable, Past Due [Line Items]
Total Past Due	31	42
≥ 90 Days Past Due and Still Accruing | Loans Acquired in a Business Combination | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Past Due	28	38
≥ 90 Days Past Due and Still Accruing | Loans Acquired in a Business Combination | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total Past Due	3	4
≥ 90 Days Past Due and Still Accruing | Loans Acquired in a Business Combination | Total consumer loans
Financing Receivable, Past Due [Line Items]
Total Past Due	6	11
≥ 90 Days Past Due and Still Accruing | Loans Acquired in a Business Combination | Direct installment
Financing Receivable, Past Due [Line Items]
Total Past Due	0	2
≥ 90 Days Past Due and Still Accruing | Loans Acquired in a Business Combination | Residential mortgages
Financing Receivable, Past Due [Line Items]
Total Past Due	4	6
≥ 90 Days Past Due and Still Accruing | Loans Acquired in a Business Combination | Consumer lines of credit
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 2	$ 3