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Investments (Narrative) (Detail) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2012
Y
Dec. 31, 2011
Y
Schedule of Investments [Line Items]    
Fair value of securities on deposit with various regulatory authorities $ 10 $ 11
Fair value of securities pledged as collateral 885.0 1,900.0
Cash pledged as collateral under investment agreements 126 224
Fair value 1,025 2,387
Gross unrealized losses 191 478
Gross unrealized losses, Investment portfolio 359 849
Weighted average contractual maturity period in years for securities in an unrealized loss position 21 21
Number of securities, unrealized loss position for continuous twelve-month period or longer 193 290
12 months or longer, unrealized losses 358 782
Rate of book value securities exceeded market value 5.00% 5.00%
Held-To-Maturity [Member]
   
Schedule of Investments [Line Items]    
Unrecognized gross losses 168 371
Unrecognized gross gains 20 17
Asset-Backed Securities [Member]
   
Schedule of Investments [Line Items]    
Fair value 273  
Gross unrealized losses 99  
Asset-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member]
   
Schedule of Investments [Line Items]    
Investments rated AAA 17.00%  
Percentage of investments rated investment grade 44.00%  
Fair value of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors 135  
Percentage of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors 49.00%  
Asset-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | 3rd Party Guarantors [Member]
   
Schedule of Investments [Line Items]    
Fair value of securities that include the benefit of guarantees provided by financial guarantors 27  
Asset-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | MBIA Corp [Member]
   
Schedule of Investments [Line Items]    
Fair value of securities that include the benefit of guarantees provided by financial guarantors 148  
Asset-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | Asset / Liability Products [Member]
   
Schedule of Investments [Line Items]    
Fair value 57  
Gross unrealized losses 52  
Mortgage-Backed Securities [Member]
   
Schedule of Investments [Line Items]    
Fair value 270  
Gross unrealized losses 52  
Mortgage-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member]
   
Schedule of Investments [Line Items]    
Investments rated AAA 18.00%  
Percentage of investments rated investment grade 62.00%  
Fair value of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors 197  
Percentage of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors 73.00%  
Mortgage-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | 3rd Party Guarantors [Member]
   
Schedule of Investments [Line Items]    
Fair value of securities that include the benefit of guarantees provided by financial guarantors 184  
Mortgage-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | MBIA Corp [Member]
   
Schedule of Investments [Line Items]    
Fair value of securities that include the benefit of guarantees provided by financial guarantors 10  
Corporate Obligations [Member]
   
Schedule of Investments [Line Items]    
Fair value 163  
Gross unrealized losses 28  
Corporate Obligations [Member] | Securities In An Unrealized Loss Position [Member]
   
Schedule of Investments [Line Items]    
Percentage of investments rated investment grade 84.00%  
Fair value of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors 14  
Percentage of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors 9.00%  
Corporate Obligations [Member] | Securities In An Unrealized Loss Position [Member] | 3rd Party Guarantors [Member]
   
Schedule of Investments [Line Items]    
Fair value of securities that include the benefit of guarantees provided by financial guarantors $ 9  
Book Value Greater Than Market Value By More Than 5% [Member]
   
Schedule of Investments [Line Items]    
Number of securities, unrealized loss position for continuous twelve-month period or longer 118 218