Investment Securities (Detail) - Two securities with credit-related deterioration evidence (ARM Senior Support 2004 [Member])	3 Months Ended
Mar. 31, 2013	Dec. 31, 2012
ARM Senior Support 2004 [Member]
Credit rating	C	C
Rating agency	Moody's	Moody's
Twelve-month average loss severity	55.72%	50.53%
Twelve-month average default rate	9.37%	11.05%
60 Day or more delinquency rate	21.35%	20.56%

Description of the credit rating indicative of likelihood of repayment of the debt instrument given by a credit rating agency (for example, Standard and Poor's, Moody's, Fitch).

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration