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Borrowings - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2007
Dec. 31, 2009
Dec. 31, 2008
Debt Instrument [Line Items]          
Medium-Term Notes matured   $ 200,000,000      
Closed a public offering Trust's fixed to floating rate preferred securities     300,000,000    
Finance lease obligation lease payment term 20 years        
Finance lease obligation 100,000,000 106,000,000      
Remaining finance lease obligation lease payment term 13 years        
Maximum issuance of unsecured Commercial Paper Notes 1,500,000,000        
Current limit for commercial paper program 800,000,000        
Maximum maturities days of Commercial Paper Notes 270 days        
Minimum level of stockholders' equity required under credit facility 5,000,000,000        
Stockholders' equity 7,714,000,000 6,226,000,000   5,073,000,000 4,061,000,000
Unsecured bank credit lines with a group of six banks 875,000,000        
Line of credit facility remaining borrowing capacity 750,000,000        
Margin requirements
         
Debt Instrument [Line Items]          
Aggregate face amount of letter of credit agreements 350,000,000        
Collateral Requirements
         
Debt Instrument [Line Items]          
Aggregate face amount of letter of credit agreements 78,000,000        
Senior Notes Due 2020
         
Debt Instrument [Line Items]          
Senior Notes maturity range, start 2014        
Senior Notes maturity range, end 2020        
Fixed interest rate, long-term debt 4.45%        
Issued Senior Notes   700,000,000      
Debt instrument maturity year 2020        
Senior Notes Due 2020 | Minimum
         
Debt Instrument [Line Items]          
Fixed interest rate, long-term debt 4.45%        
Senior Notes Due 2020 | Maximum
         
Debt Instrument [Line Items]          
Fixed interest rate, long-term debt 4.95%        
Medium Term Notes Due 2017
         
Debt Instrument [Line Items]          
Fixed interest rate, long-term debt 6.375%        
Debt instrument maturity year 2017        
Junior Subordinated Notes Due 2067
         
Debt Instrument [Line Items]          
Fixed interest rate, long-term debt 7.50%        
Fixed interest rate maturity year 2017        
Debt instrument maturity year 2067        
Credit Facility Due June 2012
         
Debt Instrument [Line Items]          
Committed, unsecured credit facility with a group of eleven banks $ 800,000,000        
Unsecured credit facility, expiration date 2012-06