Rollforward of Credit Losses Recognized in Earnings (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2011
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Beginning Balance	$ 917
Credit losses recorded
Reduction in credit losses due to redemptions	(2)
Ending Balance	$ 915

This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which a previous OTTI was recognized and the investor does not intend to sell the debt security and it is not more likely than not that the investor will be required to sell the debt security before recovery of its amortized cost basis.

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 18B

-Subparagraph e

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

-Section 50

-Paragraph 8

-Subparagraph (e)

-SubTopic 10

-Topic 320

-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27340-111563

Name:	us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsAdditionalCreditLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration