Significant Inputs Considered in Determining the Measurement of Credit Losses Recognized in Earnings for Private Residential MBS (Detail)	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Current default rate	6.00%	2.00%
Prepayment rate	7.00%	8.00%
Inputs Considered In Determining Measurement Of Credit Losses Loss Severity	61.00%	47.00%
Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Current default rate	9.00%	6.00%
Prepayment rate	8.00%	16.00%
Inputs Considered In Determining Measurement Of Credit Losses Loss Severity	74.00%	52.00%

This item represents the default rate of the underlying assets in the security that was considered as one of the potential factors in reaching a conclusion that the available-for-sale investments are other than temporarily impaired and the amount of such impairment determined to be related to credit loss recognized in earnings.

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 18A

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

Name:	us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecuritiesPortionRecognizedInEarningsNetQualitativeDisclosuresDefaultRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration