Marketable Securities (Narrative) (Details) - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 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	0	$ 1,402,000
Available-for-sale securities, gross realized gain (loss)		$ 0
Cost	3,164,000		5,195,000
Debt securities unrealized gain position
Gain (Loss) on Investments [Line Items]
Fair value	2,000,000.0		6,400,000
Accumulated pre-tax unrealized gains for marketable securities	1,000		6,000
Debt securities unrealized loss position
Gain (Loss) on Investments [Line Items]
Fair value	13,600,000		8,800,000
Accumulated pre-tax unrealized loss on marketable securities	$ 31,000		$ 36,000