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Restricted Cash, Cash Equivalents and Investments (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2013
Jun. 30, 2013
Jun. 30, 2012
Dec. 31, 2012
Dec. 31, 2012
Restricted investments
Dec. 31, 2012
Corporate securities
Restricted investments
Dec. 31, 2012
Agency securities
Restricted investments
Dec. 31, 2012
Government securities
Restricted investments
Restricted Cash, Cash Equivalents and Investments                
Restricted cash and cash equivalents $ 349 $ 349   $ 304        
Restricted investments, at fair value 0 0   121        
Restricted cash, cash equivalents and investments 349 349   425        
Available-for-sale securities, restricted                
Amortized Cost         120 30 39 51
Fair Value         121 31 39 51
Unrealized Gains         1 1    
Weighted-average remaining maturity         21 months 25 months 21 months 19 months
Realized gains from sale of available-for-sale securities 1 1 1          
Realized losses from sale of available-for-sale securities $ 1 $ 1