Mortgage Servicing Assets - Additional Information (Detail) (USD $)	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011	Mar. 31, 2011
Servicing Assets at Amortized Value [Line Items]
Interest rate for mortgage funds, min	0.27%	0.27%
Interest rate for mortgage funds, max	2.78%	2.78%
Increase (decrease) in value assigned to escrow deposits	1.00%
Increase (decrease) in the fair value of mortgage servicing assets	$ 33,000,000
Increase (decrease) in assumed default rate of commercial mortgage loans	1.00%
Increase (decrease) in the fair value of mortgage servicing assets due to change in the assumed default rate of commercial mortgage loans	2,000,000
Contractual fee income from servicing commercial mortgage loans	67,000,000	72,000,000
Escrow Deposit			$ 1,500,000,000
Minimum [Member]
Servicing Assets at Amortized Value [Line Items]
Prepayment speed, rate, max	0.00%	0.00%
Expected credit losses, rate, max	1.00%	1.00%
Residual cash flows discount rate, max	7.00%	7.00%
Maximum [Member]
Servicing Assets at Amortized Value [Line Items]
Prepayment speed, rate, max	25.00%	25.00%
Expected credit losses, rate, max	3.00%	3.00%
Residual cash flows discount rate, max	15.00%	15.00%

Contractual Fee income from servicing commercial mortgage loans.

No definition available.

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- Definition

Increase (decrease) in assumed default rate of commercial mortgage loans.

+ References

No definition available.

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- Definition

Increase (decrease) in fair value of mortgage servicing assets.

+ References

No definition available.

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- Definition

Increase (decrease) in fair value of mortgage servicing assets due to change in the assumed default rate of commercial mortgage loans.

+ References

No definition available.

+ Details

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- Definition

Increase (decrease) in value assigned to Escrow deposits.

+ References

No definition available.

+ Details

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- Definition

Mortgage servicing assets interest rate for mortgage funds maximum.

+ References

No definition available.

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- Definition

Mortgage servicing assets interest rate for mortgage funds Minimum.

+ References

No definition available.

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- Definition

The designation of funds furnished by a borrower to a lender to assure future payments of the borrower's real estate taxes and insurance obligations with respect to a mortgaged property. Escrow deposits may be made for a variety of other purposes such as earnest money and contingent payments. This element excludes replacement reserves which are an escrow separately provided for within the US GAAP taxonomy.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.10)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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- Definition

The discount rate used to estimate the fair value of servicing assets and servicing liabilities.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 140

-Paragraph 17

-Subparagraph f

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS140-4/FIN46R-8

-Paragraph B8

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

The amount of contractual collections of principal associated with a financial asset expected not to be collected due to default by the borrower which may be one of the factors applied in estimating the fair value of servicing assets.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 140

-Paragraph 17

-Subparagraph f

-Clause 2

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS140-4/FIN46R-8

-Paragraph B8

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

The estimated speed of prepayments of principal on financial assets which may be one of the factors applied in estimating the fair value of servicing assets and servicing liabilities.

+ References

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