Marketable Securities (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Maximum maturity of debt securities (less than 5 years)	5 years
Net pre-tax unrealized losses	$ 22,000	$ 19,000
Proceeds from sale of marketable securities	0	1,500,000	$ 1,500,000
Available-for-sale securities, gross realized gain (loss)	0	0
Marketable securities classified as cash equivalents, recorded basis	$ 1,600,000	$ 5,200,000