Marketable Securities (Details 2) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Cash equivalents and marketable securities, Continuous unrealized loss position
Less than one year, Fair Value	$ 125,089	$ 72,772
Unrealized gain (loss), Gross
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(15)	(21)
US Treasury and Government Short-term Debt Securities [Member]
Cash equivalents and marketable securities, Continuous unrealized loss position
Less than one year, Fair Value	27,489	18,395
Unrealized gain (loss), Gross
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2)	0
Corporate Debt Securities [Member]
Cash equivalents and marketable securities, Continuous unrealized loss position
Less than one year, Fair Value	97,600	54,377
Unrealized gain (loss), Gross
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ (13)	$ (21)