Fair Value Measurements (Details) - Recurring - Money market funds, US Government Securities and Agency Investments - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Total
Fair Value Measurements
Money market funds, U.S. Government Securities and Agency Investments	$ 119,387	$ 160,210
Level 1
Fair Value Measurements
Money market funds, U.S. Government Securities and Agency Investments	111,387	138,510
Level 2
Fair Value Measurements
Money market funds, U.S. Government Securities and Agency Investments	$ 8,000	$ 21,700