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Long-term Debt (Schedule Of Unused Portion Of Revolving Credit Facility) (Details) - USD ($)
Sep. 30, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Long-term debt $ 818,477,000 $ 602,900,000
Revolving Credit Facility [Member]    
Debt Instrument [Line Items]    
Maximum borrowing capacity under the revolving credit facility 850,000,000 850,000,000
Long-term debt 215,619,000 0
Outstanding letters of credit 2,710,000 2,830,000
Unused portion of revolving credit facility $ 631,671,000 $ 847,170,000