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Debt (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 3 Months Ended 3 Months Ended
Mar. 31, 2011
Mar. 31, 2012
Dec. 31, 2011
Mar. 30, 2011
Mar. 31, 2012
Unsecured Letter Of Credit Facility [Member]
Dec. 31, 2011
Unsecured Letter Of Credit Facility [Member]
Mar. 31, 2012
Letter Of Credit [Member]
Dec. 31, 2011
Letter Of Credit [Member]
Mar. 31, 2012
Unsecured revolving credit facility [Member]
Mar. 30, 2011
Unsecured revolving credit facility [Member]
Debt Instrument                    
Maximum borrowing capacity         $ 200.0   $ 190.0     $ 250.0
Credit Facility termination date       Mar. 30, 2011            
Credit facility expiration date         June 2014       June 2012  
Line of Credit Facility, Amount Outstanding       0            
Cash collateral for borrowed securities       250.0            
Write-off of unamortized issuance costs 1.3                  
Letters of Credit Outstanding, Amount   $ 219.3 $ 235.9   $ 148.0 $ 152.7 $ 71.3 $ 83.2