Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets measured at fair value on a recurring basis
Financial instruments, fair value	$ 17,835	$ 53,096
Level 1
Assets measured at fair value on a recurring basis
Financial instruments, fair value	8,953	6,362
Level 2
Assets measured at fair value on a recurring basis
Financial instruments, fair value	8,882	46,734
Level 3
Assets measured at fair value on a recurring basis
Financial instruments, fair value	0	0
Fair Value, Measurements, Recurring | Money Market Funds
Assets measured at fair value on a recurring basis
Cash and Cash Equivalents	8,953	6,362
Fair Value, Measurements, Recurring | Money Market Funds | Level 1
Assets measured at fair value on a recurring basis
Cash and Cash Equivalents	8,953	6,362
Fair Value, Measurements, Recurring | Money Market Funds | Level 2
Assets measured at fair value on a recurring basis
Cash and Cash Equivalents	0	0
Fair Value, Measurements, Recurring | Money Market Funds | Level 3
Assets measured at fair value on a recurring basis
Cash and Cash Equivalents	0	0
Fair Value, Measurements, Recurring | Corporate Notes and Bonds
Assets measured at fair value on a recurring basis
Marketable securities	6,389	26,278
Fair Value, Measurements, Recurring | Corporate Notes and Bonds | Level 1
Assets measured at fair value on a recurring basis
Marketable securities	0	0
Fair Value, Measurements, Recurring | Corporate Notes and Bonds | Level 2
Assets measured at fair value on a recurring basis
Marketable securities	6,389	26,278
Fair Value, Measurements, Recurring | Corporate Notes and Bonds | Level 3
Assets measured at fair value on a recurring basis
Marketable securities	0	0
Fair Value, Measurements, Recurring | U.S. Government and Agency Securities
Assets measured at fair value on a recurring basis
Marketable securities		11,976
Fair Value, Measurements, Recurring | U.S. Government and Agency Securities | Level 1
Assets measured at fair value on a recurring basis
Marketable securities		0
Fair Value, Measurements, Recurring | U.S. Government and Agency Securities | Level 2
Assets measured at fair value on a recurring basis
Marketable securities		11,976
Fair Value, Measurements, Recurring | U.S. Government and Agency Securities | Level 3
Assets measured at fair value on a recurring basis
Marketable securities		0
Fair Value, Measurements, Recurring | Commercial paper
Assets measured at fair value on a recurring basis
Marketable securities	2,493	6,982
Fair Value, Measurements, Recurring | Commercial paper | Level 1
Assets measured at fair value on a recurring basis
Marketable securities	0	0
Fair Value, Measurements, Recurring | Commercial paper | Level 2
Assets measured at fair value on a recurring basis
Marketable securities	2,493	6,982
Fair Value, Measurements, Recurring | Commercial paper | Level 3
Assets measured at fair value on a recurring basis
Marketable securities	$ 0	$ 0