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Other revenues (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Other revenues [Abstract]      
Amortization of unfavorable contracts $ 0 $ 24 $ 39
Amortization of favorable contracts (12) (23) (13)
Revenues related party 15 0 0
Total 3 1 26
Useful economic lives of unfavorable contracts, lower range 2 years    
Useful economic lives of unfavorable contracts, upper range 5 years    
Useful economic lives of favorable contracts, lower range 20 months    
Useful economic lives of favorable contracts, upper range 36 months    
Unamortized amount of favorable contracts $ 3