Marketable Securities (Details 3) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt securities, available-for-sale
Less than 12 Months, fair value	$ 254,250	$ 264,934
Less than 12 Months, gross unrealized loss	122	64
U.S. Government bonds and notes
Debt securities, available-for-sale
Less than 12 Months, fair value	72,896	14,112
Less than 12 Months, gross unrealized loss	43	1
Corporate notes, bonds and commercial paper
Debt securities, available-for-sale
Less than 12 Months, fair value	181,354	250,822
Less than 12 Months, gross unrealized loss	$ 79	$ 63