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Employee Benefits (Details 6) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2012
Level 3 [Member]
Information of plan assets using significant unobservable inputs      
Beginning balance as of December 31, 2011 $ 1,851 $ 1,505 $ 23
Capital distributions     (5)
Ending balance as of December 31, 2012 $ 1,851 $ 1,505 $ 18