Marketable Securities (Narrative) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Gain (Loss) on Investments [Line Items]
Maximum maturity of debt securities (less than 5 years)	5 years
Accumulated pre-tax unrealized gains for marketable securities	$ 157,000		$ 229,000
Proceeds from sales of available-for-sale marketable securities	0	$ 0
Cost	5,300,000		1,300,000
Unrealized gains or losses on marketable securities	$ 0		$ 0