Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011
Servicing Assets at Amortized Value [Line Items]
Balance at beginning of period	$ 173	$ 196
Servicing retained from loan sales	30	15
Purchases	31	7
Amortization	(43)	(41)
Impairments	(3)
Balance at end of period	188	177
Fair value at end of period	$ 237	$ 260

The value of mortgage-based securitized assets that were created through the writing and securitizing new loans and through purchasing previously securitized assets.

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Amortized carrying amount (balance) as of the balance sheet date of an asset representing net future revenues from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

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

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

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-Name Statement of Financial Accounting Standard (FAS)

-Number 140

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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-Name FASB Staff Position (FSP)

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 140

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-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

Fair value of the servicing asset that has been amortized as of the balance sheet date.

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- Definition

Adjustment made to the carrying value of servicing assets measured using the amortization method related to an other-than-temporary impairment, for example, due to a decrease in future servicing income or an increase in servicing costs.

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