Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets
Cash and cash equivalents	$ 51,441	$ 118,564
Cash
Assets
Cash and cash equivalents	19,018	73,302
Money market funds
Assets
Cash and cash equivalents	32,423	325
Domestic corporate commercial paper
Assets
Cash and cash equivalents		44,937
Recurring basis
Assets
Cash and cash equivalents	51,441	118,564
Marketable Securities	266,493	311,692
Recurring basis | Domestic corporate debt securities
Assets
Marketable Securities	206,881	207,086
Recurring basis | Domestic corporate commercial paper
Assets
Marketable Securities		29,837
Recurring basis | Agency bonds
Assets
Marketable Securities	59,612	74,769
Recurring basis | Cash
Assets
Cash and cash equivalents	19,018	73,302
Recurring basis | Money market funds
Assets
Cash and cash equivalents	32,423	325
Recurring basis | Domestic corporate commercial paper
Assets
Cash and cash equivalents		44,937
Recurring basis | Level 1
Assets
Cash and cash equivalents	51,441	73,627
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	19,018	73,302
Recurring basis | Level 1 | Money market funds
Assets
Cash and cash equivalents	32,423	325
Recurring basis | Level 1 | Domestic corporate commercial paper
Assets
Cash and cash equivalents		0
Recurring basis | Level 2
Assets
Cash and cash equivalents	0	44,937
Marketable Securities	266,493	311,692
Recurring basis | Level 2 | Domestic corporate debt securities
Assets
Marketable Securities	206,881	207,086
Recurring basis | Level 2 | Domestic corporate commercial paper
Assets
Marketable Securities		29,837
Recurring basis | Level 2 | Agency bonds
Assets
Marketable Securities	59,612	74,769
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 2 | Domestic corporate commercial paper
Assets
Cash and cash equivalents		44,937
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
Recurring basis | Level 3 | Domestic corporate commercial paper
Assets
Cash and cash equivalents		$ 0