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Investments (Narrative) (Detail) (USD $)
Jun. 30, 2012
Year
Dec. 31, 2011
Year
Schedule of Investments [Line Items]    
Fair value of securities on deposit with various regulatory authorities $ 10,000,000 $ 11,000,000
Fair value of securities pledged as collateral 1,000,000,000 1,900,000,000
Cash pledged as collateral under investment agreements 174,000,000 224,000,000
Fair value 1,558,000,000 2,387,000,000
Gross unrealized losses 226,000,000 478,000,000
Gross unrealized losses, Investment portfolio 386,000,000 849,000,000
Weighted average contractual maturity period in years for securities in an unrealized loss position 20 21
Number of securities, unrealized loss position for continuous twelve-month period or longer 204 290
12 months or longer, unrealized losses 381,000,000 782,000,000
Rate of book value securities exceeded market value 5.00% 5.00%
Held-To-Maturity [Member]
   
Schedule of Investments [Line Items]    
Unrecognized gross losses 160,000,000 371,000,000
Unrecognized gross gains 5,000,000 17,000,000
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 131 218
Asset-Backed Securities [Member]
   
Schedule of Investments [Line Items]    
Fair value 288,000,000  
Gross unrealized losses 119,000,000  
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 53.00%  
Fair Value of Securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors 134,000,000  
Percentage of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors 47.00%  
Asset-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | 3rd Party Guarantors [Member]
   
Schedule of Investments [Line Items]    
Benefit of guarantees provided 31,000,000  
Asset-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | MBIA Corp [Member]
   
Schedule of Investments [Line Items]    
Benefit of guarantees provided 147,000,000  
Asset-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | Asset / Liability Products [Member]
   
Schedule of Investments [Line Items]    
Fair value 65,000,000  
Gross unrealized losses 57,000,000  
Mortgage-Backed Securities [Member]
   
Schedule of Investments [Line Items]    
Fair value 284,000,000  
Gross unrealized losses 63,000,000  
Mortgage-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member]
   
Schedule of Investments [Line Items]    
Investments rated AAA 19.00%  
Percentage of investments rated investment grade 61.00%  
Fair Value of Securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors 209,000,000  
Percentage of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors 74.00%  
Mortgage-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | 3rd Party Guarantors [Member]
   
Schedule of Investments [Line Items]    
Benefit of guarantees provided 189,000,000  
Mortgage-Backed Securities [Member] | Securities In An Unrealized Loss Position [Member] | MBIA Corp [Member]
   
Schedule of Investments [Line Items]    
Benefit of guarantees provided 11,000,000  
Corporate Obligations [Member]
   
Schedule of Investments [Line Items]    
Fair value 305,000,000  
Gross unrealized losses 31,000,000  
Corporate Obligations [Member] | Securities In An Unrealized Loss Position [Member]
   
Schedule of Investments [Line Items]    
Investments rated AAA 1.00%  
Percentage of investments rated investment grade 92.00%  
Fair Value of Securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors 13,000,000  
Percentage of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors 4.00%  
Corporate Obligations [Member] | Securities In An Unrealized Loss Position [Member] | 3rd Party Guarantors [Member]
   
Schedule of Investments [Line Items]    
Benefit of guarantees provided $ 9,000,000