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Debt - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
9 Months Ended
Sep. 30, 2012
Dec. 31, 2011
Debt Instrument [Line Items]    
Accrued interest related to senior notes $ 10 $ 31
Credit Facility
   
Debt Instrument [Line Items]    
Unsecured revolving credit facility 750  
Line of credit facility, expiration date 2016-08  
Interest, on drawn amount LIBOR plus 150 basis points  
Commitment fee on undrawn amounts 0.225%  
Basis points added to LIBOR rate 1.50%  
Letters of credit issued under the credit facility 22 22
8.5% Senior Notes Due 2016
   
Debt Instrument [Line Items]    
Senior unsecured notes principal amount   400
Senior notes, interest rate   8.50%
7.456% Senior Notes Due 2018
   
Debt Instrument [Line Items]    
Senior unsecured notes principal amount 500  
Senior notes, interest rate 7.456%  
Senior notes, maturity date 2018-08  
Senior notes redemption price percentage at the option of holder 100.00%  
Redemption at option of holder repayable in whole or in part on August 15, 2013, at the option of the holders of such 7.456% Notes, at 100% of the principal amount plus accrued interest  
Redemption at option of Company We may redeem the 7.456% Notes at a redemption price of 100% of the principal plus accrued interest, plus a "make-whole" premium, in whole or in part at any time at our option.  
Fair value of senior notes 589 563
5.95% Senior Notes Due 2020
   
Debt Instrument [Line Items]    
Senior unsecured notes principal amount 750  
Senior notes, interest rate 5.95%  
Senior notes, maturity date 2020-08  
Redemption at option of Company We may redeem the 5.95% Notes at a redemption price of 100% of the principal plus accrued interest, plus a “make-whole” premium, in whole or in part at any time or from time to time at our option.  
Senior notes issued price percentage 99.893%  
Senior notes redemption price percentage at the option of the Company 100.00%  
Fair value of senior notes $ 825 $ 760