Cash Equivalents And Restricted Securities (Summary Of Cash Equivalents And Restricted Securities) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Cash Equivalents, Cost Basis	$ 27,392	$ 40,753	$ 59,595	$ 57,341
Restricted Securities, Cost Basis	808	776
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash Equivalents, Cost Basis	373	372
Restricted Securities, Cost Basis	808	776
Unrealized Holding Gains
Cash Equivalents, Estimated Market Value	373	372
Restricted Securities, Estimated Market Value	$ 808	$ 776