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Employee Benefit Plans Components of Net Periodic Benefit Cost (Credit) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2012
Jun. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Impact on accumulated other comprehensive loss due to curtailment gain       $ 21.8    
Pension and OPEB, Period Expense, Other   28.1   28.7    
Increase in Postretirement Benefits and Other Long-term Liabilities due to Reclassification 28.1     28.1    
Increase in Postretirement and Other Long-term Liabilities due to Plan Remeasurement 158.6     158.6    
Increase in GM Postretirement Cost Sharing Asset due to Plan Remeasurement 34.8     34.8    
Decrease in Accumulated Other Comprehensive Loss due to Plan Remeasurement 95.1     95.1    
Increase in Postretirement Benefits and Other Long-term Liabilities due to the Amendment of our SERP Plan 10.4     10.4    
Pension Contributions       225.0 26.2  
Pension Contributions in Excess of Statutory Minimums       114.7    
Pension Plans, Defined Benefit [Member]
            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Service cost 0.7   1.2 2.3 3.4  
Interest cost 8.9   9.2 26.5 27.6  
Expected asset return (8.0)   (7.9) (24.0) (23.7)  
Amortized loss 1.9   1.1 5.5 3.3  
Amortized prior service credit 0.1   0 0.1 0  
Net periodic benefit cost (credit) 3.6   3.6 10.4 10.6  
Regulatory Pension Funding Requirement           35
Other Postretirement Benefit Plans, Defined Benefit [Member]
            
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]            
Service cost 0.1   0.2 0.3 0.6  
Interest cost 3.8   4.3 11.6 12.9  
Amortized loss 0.1   0.1 0.5 0.3  
Amortized prior service credit (0.3)   (0.8) (1.3) (2.4)  
Curtailment gain 0   0 (21.8) 0  
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements 0 (5.2) 0 (5.2) 0  
Net periodic benefit cost (credit) 3.7   3.8 (15.9) 11.4  
Defined Benefit Plan, Settlements, Benefit Obligation   4.7        
Pension and Other Postretirement Benefits, Reduction in Accumulated Other Comprehensive Loss due to Settlement Gain   0.5        
Expected Cash Outlay for Other Postretirement Benefits           $ 16