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Significant Equity Investments - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Schedule of Equity Method Investments [Line Items]        
Income (loss) from investees $ 26 $ 48 $ 31 $ 116
Significant Equity Investments [Member]
       
Schedule of Equity Method Investments [Line Items]        
Income (loss) from investees     $ 50 $ 121