Cash Equivalents And Restricted Cash And Securities (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted securities	$ 546,000	$ 528,000
Gross realized gains or losses on cash equivalents	0	0	$ 0
Money Market Funds [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted securities	546,000	528,000
Maturity Date Within Three Months [Member] | Money Market Funds [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted securities	$ 546,000	$ 528,000
Maximum [Member] | Maturity Date Within Three Months [Member] | Money Market Funds [Member] | Measurement Input, Expected Term [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted securities maturity period	3 months	3 months