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Special Purpose Entities, Including Variable Interest Entities - Summary of Fair Value of Fixed Income Securities, by Asset-Type, Held by Consolidated Variable Interest Entities (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Variable interest entity [Member]
   
Investments:    
Fixed income securities $ 2,414,607 $ 2,261,294
Corporate obligations [Member]
   
Investments:    
Fixed income securities $ 2,414,607 $ 2,261,294