Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2013	Jun. 30, 2013	Sep. 30, 2013	Sep. 30, 2012
Servicing Asset [Abstract]
Purchases	$ 118	$ 118	$ 144	$ 31

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

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 3, 15, 16, 17, 22, 23, 147, 148, 149

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 Statement of Financial Accounting Standard (FAS)

-Number 102

-Paragraph 9

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 21

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-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 230

-Section 55

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6587932&loc=d3e60097-112785

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 17

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3367-108585

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