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Fair Value Of Financial Instruments (Narrative) (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Dec. 31, 2013
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]          
Percentage of insured credit derivatives, value under Direct Price Model and Dual-Default Model 6.00%   6.00%    
Percentage of sector-specific transactions valued using BET model 6.00%   6.00%    
Percentage of transactions valued using corporate spreads used 44.00%   44.00%    
Percentage of transactions valued using spread benchmarked from most relevant spread source 50.00%   50.00%    
Percentage of transactions valued using WARF-sourced and/or ratings sourced credit spreads 76.00%   76.00%    
Net insured derivative liability $ 284   $ 284   $ 1,200
Impact of including the nonperformance risk in valuation of pre-tax net insured derivative liability 94   94   394
Percentage of level 3 assets at fair value in total assets measured at fair value value 22.00%   22.00%   22.00%
Percentage of level 3 liabilities at fair value in total liabilities measured at fair value 41.00%   41.00%   61.00%
Transfers into Level 3 55 41 238 199  
Transfers out of Level 3 1 34 83 132  
Transfers into Level 2 1 34 83 132  
Transfers out of Level 2 55 41 238 199  
Transfers into or out of Level 1 $ 0 $ 0 $ 0 $ 0  
B E T Valuation Model [Member]
          
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]          
Percentage of insured credit derivatives, fair value using the BET model 94.00%   94.00%