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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Assets    
Cash and cash equivalents $ 62,014 $ 118,564
Cash    
Assets    
Cash and cash equivalents 15,236 73,302
Money market funds    
Assets    
Cash and cash equivalents 46,778 325
Domestic corporate commercial paper    
Assets    
Cash and cash equivalents   44,937
Recurring basis    
Assets    
Cash and cash equivalents 62,014 118,564
Marketable Securities 214,355 311,692
Recurring basis | Domestic corporate debt securities    
Assets    
Marketable Securities 154,703 207,086
Recurring basis | Domestic corporate commercial paper    
Assets    
Marketable Securities   29,837
Recurring basis | Agency bonds    
Assets    
Marketable Securities 59,652 74,769
Recurring basis | Cash    
Assets    
Cash and cash equivalents 15,236 73,302
Recurring basis | Money market funds    
Assets    
Cash and cash equivalents 46,778 325
Recurring basis | Domestic corporate commercial paper    
Assets    
Cash and cash equivalents   44,937
Recurring basis | Level 1    
Assets    
Cash and cash equivalents 62,014 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 15,236 73,302
Recurring basis | Level 1 | Money market funds    
Assets    
Cash and cash equivalents 46,778 325
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 214,355 311,692
Recurring basis | Level 2 | Domestic corporate debt securities    
Assets    
Marketable Securities 154,703 207,086
Recurring basis | Level 2 | Domestic corporate commercial paper    
Assets    
Marketable Securities   29,837
Recurring basis | Level 2 | Agency bonds    
Assets    
Marketable Securities 59,652 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