Investment Securities (Summary Of Maturity Dates Of Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less than one year	$ 23,699	$ 19,793
1-5 years	2,765	5,085
Total	26,464	24,878
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	10,140	7,490
Total	10,140	7,490
Corporate Notes And Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less than one year	13,559	12,303
1-5 years	2,765	5,085
Total	$ 16,324	$ 17,388