Marketable Securities (Marketable securities classified as available-for-sale) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 152,010	$ 135,786
Gross Unrealized Gains	47	8
Gross Unrealized Losses	(15)	(73)
Fair Value	152,042	135,721
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,441	46,660
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(36)
Fair Value	13,440	46,624
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	138,569	89,126
Gross Unrealized Gains	47	8
Gross Unrealized Losses	(14)	(37)
Fair Value	$ 138,602	$ 89,097