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Senior Notes (Schedule Of Outstanding Senior Notes) (Details) - Senior Notes [Member] - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2021
Dec. 31, 2020
4.868% Senior notes due 6/1/2044 [Member]    
Debt Instrument [Line Items]    
Date issued Jun. 05, 2014  
Date due Jun. 01, 2044 Jun. 01, 2044
Original Principal Amount $ 400,000  
Consolidated Balance Sheet Amount 397,224 $ 397,194
Market Value $ 479,616 $ 528,000
Debt instrument, stated interest rate 4.868%  
3.5% Senior notes due 10/15/2050 [Member]    
Debt Instrument [Line Items]    
Date issued Oct. 07, 2020  
Date due Oct. 15, 2050 Oct. 15, 2050
Original Principal Amount $ 1,000,000  
Consolidated Balance Sheet Amount 979,654 $ 979,524
Market Value $ 976,480 $ 1,138,100
Debt instrument, stated interest rate 3.50%