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Retirement Plans (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]    
Actual return $ 30 $ 52
Estimated future employer contributions in next fiscal year 30  
(Level 3)    
Defined Benefit Plan Disclosure [Line Items]    
Actual return $ 24