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Cash Equivalents And Restricted Securities (Summary Of Cash Equivalents And Restricted Securities) (Details) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Cash and Cash Equivalents [Line Items]        
Cash Equivalents, Cost Basis $ 35,311 $ 40,753 $ 55,013 $ 57,341
Restricted Securities, Cost Basis 753 776    
Money Market Funds [Member]        
Cash and Cash Equivalents [Line Items]        
Cash Equivalents, Cost Basis 1,237 372    
Restricted Securities, Cost Basis 753 776    
Unrealized Holding Gains    
Cash Equivalents, Estimated Market Value 1,237 372    
Restricted Securities, Estimated Market Value $ 753 $ 776