Investments (Narrative) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Accounting Standards Update No. 2016-01 [Member]
Schedule of Available-for-sale Securities [Line Items]
Reclassification of net unrealized losses from accumulated other comprehensive gain (AOCI) to retained earnings	$ 7
Minimum [Member]
Schedule of Available-for-sale Securities [Line Items]
Treasury Bills, term	1 month
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Treasury Bills, term	7 months