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Senior Notes (Narrative) (Details) - Senior Notes [Member] - 4.868% Senior notes due 6/1/2044 [Member] - USD ($)
9 Months Ended
Oct. 07, 2020
Sep. 30, 2020
Debt Instrument [Line Items]    
Date issued   Jun. 05, 2014
Debt instrument face amount   $ 400,000,000
Debt instrument, term   30 years
Debt instrument, stated interest rate   4.868%
Debt instrument, maturity date   Jun. 01, 2044
SubsequentEventMember    
Debt Instrument [Line Items]    
Date issued Oct. 07, 2020  
Debt instrument face amount $ 1,000,000,000  
Debt instrument, term 30 years  
Debt instrument, stated interest rate 3.50%  
Debt instrument, maturity date Oct. 15, 2050