Quarterly report pursuant to Section 13 or 15(d)

Fair Value (Details)

v3.8.0.1
Fair Value (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Fair Value    
Investments at fair value $ 86,628 $ 112,374
Estimate Of Fair Value Fair Value Disclosure    
Fair Value    
Investments at fair value 91,314 115,169
U.S. treasury bills    
Fair Value    
Investments at fair value 6,729 6,726
Government-sponsored enterprises securities    
Fair Value    
Investments at fair value 6,179 7,826
Corporate bonds and commercial paper    
Fair Value    
Investments at fair value 73,720 97,822
Fair value inputs Level 1    
Fair Value    
Investments at fair value 4,686 2,795
Fair value inputs Level 2    
Fair Value    
Investments at fair value 86,628 112,374
Fair value measurements recurring | Money market funds | Estimate Of Fair Value Fair Value Disclosure    
Fair Value    
Investments at fair value 4,686 2,795
Fair value measurements recurring | U.S. treasury bills | Estimate Of Fair Value Fair Value Disclosure    
Fair Value    
Investments at fair value 6,729 6,726
Fair value measurements recurring | Government-sponsored enterprises securities | Estimate Of Fair Value Fair Value Disclosure    
Fair Value    
Investments at fair value 6,179 7,826
Fair value measurements recurring | Corporate bonds and commercial paper | Estimate Of Fair Value Fair Value Disclosure    
Fair Value    
Investments at fair value 73,720 97,822
Fair value measurements recurring | Fair value inputs Level 1 | Money market funds    
Fair Value    
Investments at fair value 4,686 2,795
Fair value measurements recurring | Fair value inputs Level 2 | U.S. treasury bills    
Fair Value    
Investments at fair value 6,729 6,726
Fair value measurements recurring | Fair value inputs Level 2 | Government-sponsored enterprises securities    
Fair Value    
Investments at fair value 6,179 7,826
Fair value measurements recurring | Fair value inputs Level 2 | Corporate bonds and commercial paper    
Fair Value    
Investments at fair value $ 73,720 $ 97,822