Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets
Cash and cash equivalents	$ 66,901	$ 59,321
Marketable Securities	121,576	177,140
Domestic corporate debt securities
Assets
Marketable Securities	74,743	132,356
Domestic corporate commercial paper
Assets
Marketable Securities	16,830
Agency bonds
Assets
Marketable Securities	30,003	44,784
Cash
Assets
Cash and cash equivalents	63,660	20,448
Money market funds
Assets
Cash and cash equivalents	3,241	38,873
Recurring basis
Assets
Cash and cash equivalents	66,901	59,321
Marketable Securities	121,576	177,140
Recurring basis | Domestic corporate debt securities
Assets
Marketable Securities	74,743	132,356
Recurring basis | Domestic corporate commercial paper
Assets
Marketable Securities	16,830
Recurring basis | Agency bonds
Assets
Marketable Securities	30,003	44,784
Recurring basis | Cash
Assets
Cash and cash equivalents	63,660	20,448
Recurring basis | Money market funds
Assets
Cash and cash equivalents	3,241	38,873
Recurring basis | Level 1
Assets
Cash and cash equivalents	66,901	59,321
Marketable Securities	0	0
Recurring basis | Level 1 | Domestic corporate debt securities
Assets
Marketable Securities	0	0
Recurring basis | Level 1 | Domestic corporate commercial paper
Assets
Marketable Securities	0
Recurring basis | Level 1 | Agency bonds
Assets
Marketable Securities	0	0
Recurring basis | Level 1 | Cash
Assets
Cash and cash equivalents	63,660	20,448
Recurring basis | Level 1 | Money market funds
Assets
Cash and cash equivalents	3,241	38,873
Recurring basis | Level 2
Assets
Cash and cash equivalents	0	0
Marketable Securities	121,576	177,140
Recurring basis | Level 2 | Domestic corporate debt securities
Assets
Marketable Securities	74,743	132,356
Recurring basis | Level 2 | Domestic corporate commercial paper
Assets
Marketable Securities	16,830
Recurring basis | Level 2 | Agency bonds
Assets
Marketable Securities	30,003	44,784
Recurring basis | Level 2 | Cash
Assets
Cash and cash equivalents	0	0
Recurring basis | Level 2 | Money market funds
Assets
Cash and cash equivalents	0	0
Recurring basis | Level 3
Assets
Cash and cash equivalents	0	0
Marketable Securities	0	0
Recurring basis | Level 3 | Domestic corporate debt securities
Assets
Marketable Securities	0	0
Recurring basis | Level 3 | Domestic corporate commercial paper
Assets
Marketable Securities	0
Recurring basis | Level 3 | Agency bonds
Assets
Marketable Securities	0	0
Recurring basis | Level 3 | Cash
Assets
Cash and cash equivalents	0	0
Recurring basis | Level 3 | Money market funds
Assets
Cash and cash equivalents	$ 0	$ 0