Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
U.S treasury obligations
Assets measured at fair value on a recurring basis:
Marketable securities	$ 27,407	$ 39,844
Money market funds
Assets measured at fair value on a recurring basis:
Cash and cash equivalents	36,334	17,205
Overnight repurchase agreements
Assets measured at fair value on a recurring basis:
Cash and cash equivalents	58,000	15,000
Recurring
Assets measured at fair value on a recurring basis:
Total	121,741	72,049
Recurring \| U.S treasury obligations
Assets measured at fair value on a recurring basis:
Marketable securities	27,407	39,844
Recurring \| Money market funds
Assets measured at fair value on a recurring basis:
Cash and cash equivalents	36,334	17,205
Recurring \| Overnight repurchase agreements
Assets measured at fair value on a recurring basis:
Cash and cash equivalents	58,000	15,000
Recurring \| Level 1
Assets measured at fair value on a recurring basis:
Total	63,741	57,049
Recurring \| Level 1 \| U.S treasury obligations
Assets measured at fair value on a recurring basis:
Marketable securities	27,407	39,844
Recurring \| Level 1 \| Money market funds
Assets measured at fair value on a recurring basis:
Cash and cash equivalents	36,334	17,205
Recurring \| Level 2
Assets measured at fair value on a recurring basis:
Total	58,000	15,000
Recurring \| Level 2 \| Overnight repurchase agreements
Assets measured at fair value on a recurring basis:
Cash and cash equivalents	$ 58,000	$ 15,000