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Restricted Cash, Cash Equivalents and Investments (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2012
Dec. 31, 2011
Restricted Cash, Cash Equivalents and Investments    
Restricted cash and cash equivalents $ 310 $ 348
Restricted investments, at fair value 205 226
Restricted cash, cash equivalents and investments 515 574
Available-for-sale securities, restricted    
Realized gains from sale of available-for-sale securities 1  
Restricted investments
   
Available-for-sale securities, restricted    
Amortized Cost 203 223
Fair Value 205 226
Unrealized Gains 2 3
Weighted-average remaining maturity (in months) 24 25
Corporate securities | Restricted investments
   
Available-for-sale securities, restricted    
Amortized Cost 50 53
Fair Value 51 54
Unrealized Gains 1 1
Weighted-average remaining maturity (in months) 29 28
Agency securities | Restricted investments
   
Available-for-sale securities, restricted    
Amortized Cost 89 118
Fair Value 90 119
Unrealized Gains 1 1
Weighted-average remaining maturity (in months) 21 19
Government securities | Restricted investments
   
Available-for-sale securities, restricted    
Amortized Cost 64 52
Fair Value 64 53
Unrealized Gains   $ 1
Weighted-average remaining maturity (in months) 25 34