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Long-term Debt (Schedule Of Unused Portion Of Revolving Credit Facility) (Details) (USD $)
Mar. 31, 2013
Dec. 31, 2012
Mar. 14, 2012
Dec. 31, 2011
Debt Instrument [Line Items]        
Outstanding borrowings $ 430,615,000 $ 428,789,000    
Revolving Credit Facility [Member]
       
Debt Instrument [Line Items]        
Maximum borrowing capacity under the revolving credit facility 650,000,000 650,000,000.0 650,000,000 300,000,000
Outstanding borrowings 76,000,000 75,000,000    
Outstanding letters of credit 3,100,000 3,100,000    
Unused portion of revolving credit facility $ 570,900,000 $ 571,900,000