Securities - Additional Information (Detail) $ in Millions	9 Months Ended
Sep. 30, 2017 USD ($) MortgageLoan position	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Pledged available for sale and held-to-maturity securities	$ 7,800
Agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized loss attributable to mortgage-backed securities (less than)	1
Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses	$ 203	$ 275
Number of fixed rate collateralized mortgage obligations \| MortgageLoan	352
Weighted-average maturity collateralized mortgage, in years	4 years 1 month 13 days
Residential Mortgage Backed Securities [Member] \| Agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses	$ 10	$ 17
Number of other mortgage-backed securities positions \| position	219
Weighted-average maturity mortgage-backed securities, in years	3 years 11 months 4 days

Number of fixed-rate collateralized mortgage obligations.

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Weighted average maturity mortgage backed securities in years.

+ References

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Weighted Average Maturity Mortgage Backed Securities

+ References

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- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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Amount of unrealized loss on investments in available-for-sale securities that have been in a loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

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- Definition

Carrying amount as of the balance sheet date of loans that have been pledged as collateral for borrowings. If the secured party has the right by contract or custom to sell or re-pledge the collateral, then the debtor reclassifies that asset and report that asset in its statement of financial position separately from other assets not so encumbered.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5086-111524

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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