Marketable Securities (Marketable securities classified as available-for-sale) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 111,515	$ 91,093
Gross Unrealized Gains	22	43
Gross Unrealized Losses	(22)	(10)
Fair Value	111,515	91,126
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	33,872	23,636
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(12)	(4)
Fair Value	33,860	23,632
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	77,643	67,457
Gross Unrealized Gains	22	43
Gross Unrealized Losses	(10)	(6)
Fair Value	$ 77,655	$ 67,494