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Derivatives Foreign Currency Derivatives (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Derivative [Line Items]    
Total GM Postretirement Cost Sharing Asset $ 273.0 $ 270.6
Forward Contracts [Member]
   
Derivative [Line Items]    
Derivative, Notional Amount $ 31.9 $ 68.6