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Fair Value Measurements and Derivatives - Changes in Level 3 Recurring Fair Value Measurements (Details) (Notes Receivable, USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Notes Receivable
   
Fair Value Measurements [Line Items]    
Beginning of period $ 129 $ 116
Accretion of value (interest income) 4 4
Other    (1)
End of period $ 133 $ 119