Marketable Securities (Details 2) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Cash equivalents and marketable securities, Continuous unrealized loss position
Less than one year, Fair Value	$ 159,079	$ 216,628
Unrealized gain (loss), Gross
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(43)	(101)
US Treasury and Government Short-term Debt Securities [Member]
Cash equivalents and marketable securities, Continuous unrealized loss position
Less than one year, Fair Value	4,243	32,630
Unrealized gain (loss), Gross
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1)	(4)
Corporate Debt Securities [Member]
Cash equivalents and marketable securities, Continuous unrealized loss position
Less than one year, Fair Value	154,836	183,998
Unrealized gain (loss), Gross
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ (42)	$ (97)