Marketable Securities (Marketable securities classified as available-for-sale) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 180,184	$ 137,569
Gross Unrealized Gains	169	29
Gross Unrealized Losses	(18)	(50)
Fair Value	180,335	137,548
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,066	0
Gross Unrealized Gains	12	0
Gross Unrealized Losses	0	0
Fair Value	15,078	0
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	43,152	20,748
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(2)	(9)
Fair Value	43,151	20,739
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	121,966	116,821
Gross Unrealized Gains	156	29
Gross Unrealized Losses	(16)	(41)
Fair Value	$ 122,106	$ 116,809