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Commitments and Contingent Liabilities (Details) (USD $)
In Millions, unless otherwise specified
 6 Months Ended 12 Months Ended
Jun. 30, 2013
Dec. 31, 2012
Credit Facility One Hundred Million [Member]
    
Line Of Credit Facility [Line Items]    
Facility Capacity $ 100.0  
Maturity Date June 2017  
Amount Utilized 90.3 [1] 0 [1]
Basis Of Fees Fixed  
Credit Facility 270 Million [Member]
    
Line Of Credit Facility [Line Items]    
Facility Capacity 270.0  
Maturity Date November 2017  
Amount Utilized 270.0 [1] 0 [1]
Basis Of Fees Fixed  
Credit Facility 120 Million [Member]
    
Line Of Credit Facility [Line Items]    
Facility Capacity 120.0  
Maturity Date May 2016  
Amount Utilized 80.0 [1] 100.0 [1]
Basis Of Fees Fixed  
Credit Facility 200 Million [Member]
    
Line Of Credit Facility [Line Items]    
Facility Capacity 200.0  
Maturity Date September 2019  
Amount Utilized 200.0 [1] 200.0 [1]
Basis Of Fees Fixed  
Credit Facility 850 Million [Member]
    
Line Of Credit Facility [Line Items]    
Facility Capacity 850.0  
Maturity Date December 2015  
Amount Utilized $ 94.2 [1] $ 402.9 [1]
Basis Of Fees Senior unsecured long-term debt rating  
[1] Represents issued but undrawn letters of credit. There was no cash borrowed for the periods presented.