Marketable Securities (Narrative) (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
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	$ 74,000		$ 74,000
Accumulated pre-tax unrealized losses on marketable securities					$ 54,000
Proceeds from sales of available-for-sale marketable securities	0	$ 0	0	$ 70,000
Available-for-sale securities, gross realized gain (loss)				$ 0
Cost	$ 5,300,000		5,300,000		2,500,000
Unrealized gains or losses on marketable securities			$ 0		$ 0