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Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments, by Contractual Maturity (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost, Due in one year or less $ 906,835  
Amortized Cost, Due after one year through five years 916,863  
Amortized Cost, Due after five years through ten years 1,035,633  
Amortized Cost, Due after ten years 888,233  
Amortized Cost, Total 3,747,564  
Amortized Cost 5,653,081 5,678,560
Estimated Fair Value, Due in one year or less 910,005  
Estimated Fair Value, Due after one year through five years 979,204  
Estimated Fair Value, Due after five years through ten years 1,156,493  
Estimated Fair Value, Due after ten years 978,051  
Estimated Fair Value due, Total 4,023,753  
Estimated Fair Value 6,398,450 6,329,832
Residential mortgage-backed securities [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 1,085,177  
Estimated Fair Value 1,536,889  
Collateralized debt obligations [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 26,598  
Estimated Fair Value 27,357  
Other asset-backed securities [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 793,742  
Estimated Fair Value $ 810,451