Securities - Additional Information (Detail) (USD $)	3 Months Ended
Mar. 31, 2015 MortgageLoan	Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized loss attributable to collateralized mortgage obligations	$ 77,000,000
Number of fixed rate collateralized mortgage obligations	56
Weighted-average maturity collateralized mortgage, in years	3 years 10 months 24 days
Impairment recognized in earnings		0
Pledged available for sale and held-to-maturity securities	8,000,000,000us-gaap_LoansPledgedAsCollateral
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Impairment recognized in earnings	$ 1,000,000

Gross unrealized losses attributable to collateralized mortgage obligations.

No definition available.

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- Definition

Number of fixed-rate collateralized mortgage obligations.

+ References

No definition available.

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- Definition

Weighted average maturity collateralized mortgage life.

+ References

No definition available.

+ Details

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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 310

-SubTopic 10

-Section 50

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5086-111524

Reference 2: 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=6467456&loc=d3e113888-111728

+ Details

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- Definition

The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 8B

-URI http://asc.fasb.org/extlink&oid=27724398&loc=SL6284393-111563

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- Details

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