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Commitments and Contingent Liabilities (Details) (USD $)
 3 Months Ended 12 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Credit Facility 850 Million [Member]
    
Line Of Credit Facility [Line Items]    
Credit Facility Maturity Date December 2015  
Credit Facility Basis Of Fees Senior unsecured long-term debt rating  
Credit Facility Amount Utilized $ 140,000 $ 402,900
Credit Facility 200 Million [Member]
    
Line Of Credit Facility [Line Items]    
Credit Facility Maturity Date September 2019  
Credit Facility Basis Of Fees Fixed  
Credit Facility Amount Utilized 200,000 200,000
Credit Facility 120 Million [Member]
    
Line Of Credit Facility [Line Items]    
Credit Facility Maturity Date May 2016  
Credit Facility Basis Of Fees Fixed  
Credit Facility Amount Utilized 100,000 100,000
Credit Facility 270 Million [Member]
    
Line Of Credit Facility [Line Items]    
Credit Facility Maturity Date November 2017  
Credit Facility Basis Of Fees Fixed  
Credit Facility Amount Utilized $ 270,000 $ 0