Quantitative Information About Level 3 Fair Value Measurements (Detail) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2012
Dec. 31, 2011
Sep. 30, 2012
Fixed maturity securities available for sale
Dec. 31, 2011
Fixed maturity securities available for sale
Sep. 30, 2012
Level 3
Dec. 31, 2011
Level 3
Sep. 30, 2012
Level 3
Fixed maturity securities available for sale
Sep. 30, 2012
Level 3
Minimum
Fixed maturity securities available for sale
Sep. 30, 2012
Level 3
Maximum
Fixed maturity securities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                  
Fixed maturity securities $ 2,916,078 $ 2,718,831 $ 2,878,625 $ 2,682,142 $ 53,001 $ 44,882 $ 51,792    
Fair Value of Fixed maturity securities on Valuation Technique             Market comparable    
Fair Value Measurements Unobservable Inputs             Spread adjustment    
Fair value inputs spread adjustment, range               0.69% 3.53%
Fair value input Spread adjustment percentage of Weighted Average for Fair Value Measurement             2.04%