Loans and Leases and the Allowance for Credit Losses - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Loans And Leases Receivable [Line Items]
Commercial real estate loans held for sale	$ 381	$ 22
Contractual principal and interest payments	$ 326	410
Minimum delinquency period for loan level collectability analysis consumer mortgage	150 days
Amount of foreclosed residential real estate property held	$ 85	108
Loans secured by residential real estate that were in the process of foreclosure	$ 404	497
Percentage loans in the process of foreclosure, serviced by other entities, classified as purchased impaired	40.00%
Percentage loans in the process of foreclosure, serviced by other entities, classified as government guaranteed	20.00%
Residential Mortgage Loans [Member]
Loans And Leases Receivable [Line Items]
Mortgage loans held for sale	$ 258	356
Carrying value of loans for which partial charge-off has been recognized	29	34
Mortgage loans on real real estate not requiring charge off due to collateral carrying amount	18	20
Home Equity Loans and Lines of Credit [Member]
Loans And Leases Receivable [Line Items]
Carrying value of loans for which partial charge-off has been recognized	23	25
Mortgage loans on real real estate not requiring charge off due to collateral carrying amount	$ 29	$ 32
Maximum [Member]
Loans And Leases Receivable [Line Items]
Purchased impaired loans as a percentage of total assets	1.00%	1.00%
Minimum [Member]
Loans And Leases Receivable [Line Items]
Loan delinquent period	90 days

The threshold number of days in delinquency at which criticized loans are generally classified as non-accrual.

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- Definition

The amount of purchased impaired loans as a percentage of total assets.

+ References

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- Definition

Loans and leases receivable.

+ References

No definition available.

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- Definition

Loans and leases receivable net of partial charge off.

+ References

No definition available.

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- Definition

Minimum delinquency period threshold at which residential real estate loans and outstanding balances of home equity lines of credit are generally evaluated for collectability on a loan-by-loan basis giving consideration to estimated collateral values.

+ References

No definition available.

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- Definition

The carrying amount of the outstanding balance due under mortgage loans that were more than 150 days past due but did not require a partial charge off because the net realizable value of the collateral exceeded the outstanding customer balance.

+ References

No definition available.

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- Definition

Percentage of loans in the process of foreclosure, serviced by other entities, classified as government guaranteed.

+ References

No definition available.

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- Definition

Percentage of loans in the process of foreclosure, serviced by other entities, classified as purchased impaired.

+ References

No definition available.

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- Definition

Residential mortgage loans secured by residential real estate in process of foreclosure.

+ References

No definition available.

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- Definition

Fair value at acquisition of acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 15
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82894507&loc=d3e13503-111538

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

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- Definition

Amount, after allowance for bad debt, of commercial mortgage loan receivables held-for-sale that are not part of a disposal group. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.

+ References

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=82891408&loc=d3e4428-111522

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- Definition

Amount, after allowance for bad debt, of mortgage loan receivables held-for-sale that are not part of a disposal group. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.

+ References

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- Definition

Carrying amount as of the balance sheet date of other real estate and foreclosed assets. Other real estate may include real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned. Foreclosed assets include all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10(4))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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