Marketable Securities (Details 2) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Cash equivalents and marketable securities, Continuous unrealized loss position
Less than one year, Fair Value	$ 220,339	$ 216,628
Unrealized gain (loss), Gross
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(66)	(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	36,490	32,630
Unrealized gain (loss), Gross
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(3)	(4)
Corporate Debt Securities [Member]
Cash equivalents and marketable securities, Continuous unrealized loss position
Less than one year, Fair Value	183,849	183,998
Unrealized gain (loss), Gross
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ (63)	$ (97)