Investment Securities (Summary Of Maturity Dates Of Investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less than one year	$ 19,793	$ 23,451
1-5 years	5,085	8,629
Total	24,878	32,080
Commercial Paper Original Maturities of 91 Days or More [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less than one year	7,490	8,461
Total	7,490	8,461
Corporate Notes And Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less than one year	12,303	14,990
1-5 years	5,085	8,629
Total	$ 17,388	$ 23,619