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Senior Notes (Schedule Of Outstanding Senior Notes) (Details) - USD ($)
3 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Debt Instrument [Line Items]    
Consolidated Balance Sheet Amount $ 396,984,000 $ 396,954,000
4.868% Senior notes due 6/1/2044 [Member] | Senior Notes [Member]    
Debt Instrument [Line Items]    
Date issued Jun. 05, 2014  
Date due Jun. 01, 2044  
Principal Amounts $ 400,000,000  
Consolidated Balance Sheet Amount 396,984,000 396,954,000
Market Value $ 408,224,000 $ 396,968,000