Marketable Securities (Narrative) (Details) - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
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		$ 12,000
Accumulated pre-tax unrealized loss on marketable securities				$ 19,000
Proceeds from sales of available-for-sale marketable securities	$ 625,000	0	$ 1,402,000
Available-for-sale securities, gross realized gain (loss)	$ 0		$ 0
Cost		4,402,000		5,195,000
Debt securities unrealized gain position
Gain (Loss) on Investments [Line Items]
Fair value		2,500,000		6,400,000
Accumulated pre-tax unrealized gains for marketable securities		2,000		6,000
Debt securities unrealized loss position
Gain (Loss) on Investments [Line Items]
Fair value		12,900,000		8,800,000
Accumulated pre-tax unrealized loss on marketable securities		$ 34,000		$ 36,000