Marketable Securities - Carrying Amounts of Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 41,037	$ 30,002	$ 20,014
Long-term marketable securities	20,000	36,098	61,191
Total marketable securities	61,037	66,100	81,205
U.S. Government and agency obligations [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	41,037	24,996	15,000
Long-term marketable securities	20,000	36,098	61,191
Corporate bonds [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 0	$ 5,006	$ 5,014