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Fair Value of Financial Instruments (Details) - Fair Value Measurements, Recurring Basis - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Assets:    
Cash and cash equivalents $ 72,700 $ 50,800
Total assets measured at fair value 185,507 251,666
Quoted Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Total assets measured at fair value 72,744 46,781
Significant Other Observable Inputs (Level 2)    
Assets:    
Total assets measured at fair value 112,763 204,885
Significant Unobservable Inputs (Level 3)    
Assets:    
Total assets measured at fair value 0 0
Money market funds    
Assets:    
Cash and cash equivalents 72,744 46,781
Money market funds | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Cash and cash equivalents 72,744 46,781
Money market funds | Significant Other Observable Inputs (Level 2)    
Assets:    
Cash and cash equivalents 0 0
Money market funds | Significant Unobservable Inputs (Level 3)    
Assets:    
Cash and cash equivalents 0 0
U.S. Treasury securities    
Assets:    
Available-for-sale securities 12,759 8,826
U.S. Treasury securities | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Available-for-sale securities 0 0
U.S. Treasury securities | Significant Other Observable Inputs (Level 2)    
Assets:    
Available-for-sale securities 12,759 8,826
U.S. Treasury securities | Significant Unobservable Inputs (Level 3)    
Assets:    
Available-for-sale securities 0 0
Government-sponsored enterprises    
Assets:    
Available-for-sale securities 22,983 29,759
Government-sponsored enterprises | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Available-for-sale securities 0 0
Government-sponsored enterprises | Significant Other Observable Inputs (Level 2)    
Assets:    
Available-for-sale securities 22,983 29,759
Government-sponsored enterprises | Significant Unobservable Inputs (Level 3)    
Assets:    
Available-for-sale securities 0 0
Corporate debt securities    
Assets:    
Available-for-sale securities 77,021 166,300
Corporate debt securities | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Available-for-sale securities 0 0
Corporate debt securities | Significant Other Observable Inputs (Level 2)    
Assets:    
Available-for-sale securities 77,021 166,300
Corporate debt securities | Significant Unobservable Inputs (Level 3)    
Assets:    
Available-for-sale securities $ 0 $ 0