Allowance for Credit Losses - Summary of Individual and Collective Allowance for Credit Losses and Loan and Lease Balances by Class (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Valuation Allowance [Line Items]
Total Loans and Leases	$ 23,289	$ 22,153
Total commercial loans and leases
Valuation Allowance [Line Items]
Total Loans and Leases	14,721	13,761
Commercial real estate
Valuation Allowance [Line Items]
Total Loans and Leases	8,960	8,786
Commercial and industrial
Valuation Allowance [Line Items]
Total Loans and Leases	5,308	4,556
Commercial leases
Valuation Allowance [Line Items]
Total Loans and Leases	432	373
Other
Valuation Allowance [Line Items]
Total Loans and Leases	21	46
Total consumer loans
Valuation Allowance [Line Items]
Total Loans and Leases	8,568	8,392
Direct installment
Valuation Allowance [Line Items]
Total Loans and Leases	1,821	1,764
Residential mortgages
Valuation Allowance [Line Items]
Total Loans and Leases	3,374	3,113
Indirect installment
Valuation Allowance [Line Items]
Total Loans and Leases	1,922	1,933
Consumer lines of credit
Valuation Allowance [Line Items]
Total Loans and Leases	1,451	1,582
Originated Loans and Leases
Valuation Allowance [Line Items]
Allowance, individually evaluated for impairment	4	4
Allowance, collectively evaluated for impairment	185	173
Total Loans and Leases	20,397	18,062
Loans and leases outstanding, individually evaluated for impairment	30	18
Loans and leases outstanding, collectively evaluated for impairment	20,367	18,044
Originated Loans and Leases \| Total commercial loans and leases
Valuation Allowance [Line Items]
Allowance, individually evaluated for impairment	4	4
Allowance, collectively evaluated for impairment	122	115
Total Loans and Leases	12,630	10,730
Loans and leases outstanding, individually evaluated for impairment	30	18
Loans and leases outstanding, collectively evaluated for impairment	12,600	10,712
Originated Loans and Leases \| Commercial real estate
Valuation Allowance [Line Items]
Allowance, individually evaluated for impairment	2	0
Allowance, collectively evaluated for impairment	58	55
Total Loans and Leases	7,114	6,171
Loans and leases outstanding, individually evaluated for impairment	13	7
Loans and leases outstanding, collectively evaluated for impairment	7,101	6,164
Originated Loans and Leases \| Commercial and industrial
Valuation Allowance [Line Items]
Allowance, individually evaluated for impairment	2	4
Allowance, collectively evaluated for impairment	51	49
Total Loans and Leases	5,063	4,140
Loans and leases outstanding, individually evaluated for impairment	17	11
Loans and leases outstanding, collectively evaluated for impairment	5,046	4,129
Originated Loans and Leases \| Commercial leases
Valuation Allowance [Line Items]
Allowance, individually evaluated for impairment	0	0
Allowance, collectively evaluated for impairment	11	9
Total Loans and Leases	432	373
Loans and leases outstanding, individually evaluated for impairment	0	0
Loans and leases outstanding, collectively evaluated for impairment	432	373
Originated Loans and Leases \| Other
Valuation Allowance [Line Items]
Allowance, individually evaluated for impairment	0	0
Allowance, collectively evaluated for impairment	2	2
Total Loans and Leases	21	46
Loans and leases outstanding, individually evaluated for impairment	0	0
Loans and leases outstanding, collectively evaluated for impairment	21	46
Originated Loans and Leases \| Total consumer loans
Valuation Allowance [Line Items]
Allowance, individually evaluated for impairment	0	0
Allowance, collectively evaluated for impairment	63	58
Total Loans and Leases	7,767	7,332
Loans and leases outstanding, individually evaluated for impairment	0	0
Loans and leases outstanding, collectively evaluated for impairment	7,767	7,332
Originated Loans and Leases \| Direct installment
Valuation Allowance [Line Items]
Allowance, individually evaluated for impairment	0	0
Allowance, collectively evaluated for impairment	13	14
Total Loans and Leases	1,758	1,668
Loans and leases outstanding, individually evaluated for impairment	0	0
Loans and leases outstanding, collectively evaluated for impairment	1,758	1,668
Originated Loans and Leases \| Residential mortgages
Valuation Allowance [Line Items]
Allowance, individually evaluated for impairment	0	0
Allowance, collectively evaluated for impairment	22	19
Total Loans and Leases	2,995	2,612
Loans and leases outstanding, individually evaluated for impairment	0	0
Loans and leases outstanding, collectively evaluated for impairment	2,995	2,612
Originated Loans and Leases \| Indirect installment
Valuation Allowance [Line Items]
Allowance, individually evaluated for impairment	0	0
Allowance, collectively evaluated for impairment	19	15
Total Loans and Leases	1,922	1,933
Loans and leases outstanding, individually evaluated for impairment	0	0
Loans and leases outstanding, collectively evaluated for impairment	1,922	1,933
Originated Loans and Leases \| Consumer lines of credit
Valuation Allowance [Line Items]
Allowance, individually evaluated for impairment	0	0
Allowance, collectively evaluated for impairment	9	10
Total Loans and Leases	1,092	1,119
Loans and leases outstanding, individually evaluated for impairment	0	0
Loans and leases outstanding, collectively evaluated for impairment	$ 1,092	$ 1,119

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

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Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

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-Topic 310
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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
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Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118952595&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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