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Cash Equivalents And Restricted Cash And Securities (Summary Of Cash Equivalents And Restricted Securities) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Cash and Cash Equivalents [Line Items]        
Cash Equivalents, Cost Basis $ 61,958 $ 34,294 $ 42,236 $ 40,753
Cost Basis 12,030 2,000    
Restricted Securities, Cost Basis 546 528    
Unrealized Holding Gains    
Estimated Market Value 12,030 2,000    
Money Market Funds [Member]        
Cash and Cash Equivalents [Line Items]        
Cash Equivalents, Cost Basis 11,484 1,472    
Restricted Securities, Cost Basis 546 528    
Unrealized Holding Gains    
Cash Equivalents, Estimated Market Value 11,484 1,472    
Restricted Securities, Estimated Market Value $ 546 $ 528