Reconciliation of Credit Losses Recognized in Earnings on Fixed Maturity Securities Held by Company (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2012
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]    
Credit losses on securities held at beginning of the period in accumulated other comprehensive income $ 13,857 $ 13,559
Additions for credit losses not previously recognized in other-than-temporary impairment   29
Additions for increases in the credit loss for which an other-than-temporary impairment was previously recognized when there was no intent to sell the security before recovery of its amortized cost basis 650 927
Reductions for securities sold during the period (realized)      
Reductions for securities previously recognized in other comprehensive income because of intent to sell the security before recovery of its amortized cost basis      
Reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security (5) (13)
Credit losses on securities held at the end of the period in accumulated other comprehensive income $ 14,502 $ 14,502