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Long-term Debt (Schedule Of Unused Portion Of Revolving Credit Facility) (Details) - USD ($)
Mar. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]    
Long-term debt $ 541,124,000 $ 490,920,000
Revolving Credit Facility [Member]    
Debt Instrument [Line Items]    
Maximum borrowing capacity under the revolving credit facility 650,000,000 650,000,000
Long-term debt 190,000,000 145,000,000
Outstanding letters of credit 3,685,000 2,895,000
Unused portion of revolving credit facility $ 456,315,000 $ 502,105,000