Marketable Securities (Marketable securities classified as available-for-sale) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 91,093	$ 137,569
Gross Unrealized Gains	43	29
Gross Unrealized Losses	(10)	(50)
Fair Value	91,126	137,548
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	23,636	20,748
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(4)	(9)
Fair Value	23,632	20,739
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	67,457	116,821
Gross Unrealized Gains	43	29
Gross Unrealized Losses	(6)	(41)
Fair Value	$ 67,494	$ 116,809