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Properties And Equipment (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Properties And Equipment [Abstract]      
Net book value of computer software $ 8.9 $ 9.6  
Depreciation expense for computer software $ 5.5 $ 4.6 $ 5.4