Cash Equivalents And Restricted Securities (Summary Of Cash Equivalents And Restricted Securities) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Cash and Cash Equivalents [Line Items]
Cash Equivalents, Cost Basis	$ 51,027	$ 61,958	$ 126,118	$ 34,294
Cost Basis	10,559	12,030
Restricted Securities, Cost Basis	554	546
Unrealized Holding Gains
Estimated Market Value	10,559	12,030
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash Equivalents, Cost Basis	10,005	11,484
Restricted Securities, Cost Basis	554	546
Unrealized Holding Gains
Cash Equivalents, Estimated Market Value	10,005	11,484
Restricted Securities, Estimated Market Value	$ 554	$ 546