Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financial Assets:
Financial Assets, fair value	$ 243,569	$ 241,749
Financial Liabilities:
Financial Liabilities, fair value	978	998
Contingent Consideration Related To Acquisition
Financial Liabilities:
Financial Liabilities, fair value	978	998
Money Market Funds
Financial Assets:
Financial Assets, fair value	30,305	22,082
U.S. Government and Agency Securities
Financial Assets:
Financial Assets, fair value	56,228	59,001
Corporate Debt Securities
Financial Assets:
Financial Assets, fair value	66,667	70,964
Commercial Paper
Financial Assets:
Financial Assets, fair value	90,369	89,702
Level 1
Financial Assets:
Financial Assets, fair value	30,305	22,082
Level 1 | Money Market Funds
Financial Assets:
Financial Assets, fair value	30,305	22,082
Level 2
Financial Assets:
Financial Assets, fair value	213,264	219,667
Level 2 | U.S. Government and Agency Securities
Financial Assets:
Financial Assets, fair value	56,228	59,001
Level 2 | Corporate Debt Securities
Financial Assets:
Financial Assets, fair value	66,667	70,964
Level 2 | Commercial Paper
Financial Assets:
Financial Assets, fair value	90,369	89,702
Level 3
Financial Liabilities:
Financial Liabilities, fair value	978	998
Level 3 | Contingent Consideration Related To Acquisition
Financial Liabilities:
Financial Liabilities, fair value	$ 978	$ 998