Fair Value Measurements (Details) - Money market funds, US Government Securities and Agency Investments - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Total
Fair Value Measurements
Money market funds, U.S. Government Securities and Agency Investments	$ 160,210	$ 116,433
Level 1
Fair Value Measurements
Money market funds, U.S. Government Securities and Agency Investments	138,510	$ 116,433
Recurring | Level 2
Fair Value Measurements
Money market funds, U.S. Government Securities and Agency Investments	$ 21,700