Marketable Securities (Narrative) (Details) - USD ($)	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Gain (Loss) on Investments [Line Items]
Maximum maturity of debt securities (less than 5 years)		5 years
Accumulated pre-tax unrealized loss on marketable securities	$ 112,000	$ 112,000		$ 22,000
Proceeds from sales of available-for-sale marketable securities		480,000	$ 0
Available-for-sale securities, gross realized gain (loss)	3,000	3,000	0
Proceeds from sales of available-for-sale marketable securities			$ 0
Cost	$ 2,300,000	2,300,000		1,600,000
Unrealized gains or losses on marketable securities		$ 0		$ 0