Marketable Securities (Narrative) (Details) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
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	$ 287,000		$ 84,000
Proceeds from sales of available-for-sale marketable securities	0	$ 0
Cost	1,900,000		2,600,000
Unrealized gains or losses on marketable securities	$ 0		$ 0