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Line Of Credit And Capital Lease Obligations (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
Oct. 31, 2011
Line of Credit Facility [Line Items]    
Line of Credit facility, borrowing capacity $ 500 $ 350
Line of Credit facility, maturity date October 2016  
Line of Credit facility, estimated increase in borrowings 570  
Line of Credit facility, outstanding principal balance 29.0  
Line of Credit facility, reduction in the available balance 5.5  
Letters of credit, weighted average interest rate 1.00%  
Line of Credit facility, available balance 465.5  
Letters Of Credit [Member]
    
Line of Credit Facility [Line Items]    
Line of Credit facility, borrowing capacity 50  
Swingline Loans [Member]
    
Line of Credit Facility [Line Items]    
Line of Credit facility, borrowing capacity $ 25  
Federal Funds Rate [Member]
    
Line of Credit Facility [Line Items]    
Debt instrument, interest rate 0.50%  
Maximum [Member] | LIBOR [Member]
    
Line of Credit Facility [Line Items]    
Debt instrument, interest rate, as defined in the Line of Credit 1.00%  
Applicable Margin Rate [Member] | Maximum [Member] | LIBOR [Member]
    
Line of Credit Facility [Line Items]    
Debt instrument, interest rate, as defined in the Line of Credit 0.50%  
Applicable Margin Rate [Member] | Minimum [Member] | LIBOR [Member]
    
Line of Credit Facility [Line Items]    
Debt instrument, interest rate, as defined in the Line of Credit 0.125%  
Alternate Base Rate [Member] | Maximum [Member] | LIBOR [Member]
    
Line of Credit Facility [Line Items]    
Debt instrument, interest rate, as defined in the Line of Credit 1.50%  
Alternate Base Rate [Member] | Minimum [Member] | LIBOR [Member]
    
Line of Credit Facility [Line Items]    
Debt instrument, interest rate, as defined in the Line of Credit 1.125%