Summary of Cumulative Amounts Related to Credit Loss Portion of the OTTI Losses on Debt Securities (Detail) (USD $) In Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Beginning balance	$ 1,885	$ 2,731	$ 1,658	$ 2,523
Additions for credit loss impairments recognized in the current period on securities not previously impaired	623	309	850	517
Additions for credit loss impairments recognized in the current period on securities previously impaired
Reductions for credit loss impairments previously recognized on securities sold during the period		(935)		(935)
Ending balance	$ 2,508	$ 2,105	$ 2,508	$ 2,105

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.

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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 no such other than temporary impairment (OTTI) was previously recognized.

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The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity).

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This item represents reductions in the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security because of the sale of impaired securities during the period.

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