Quarterly report pursuant to Section 13 or 15(d)

Fair Value (Details)

v3.8.0.1
Fair Value (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Fair Value    
Investments at fair value $ 62,592 $ 65,030
Estimate Of Fair Value Fair Value Disclosure    
Fair Value    
Investments at fair value 67,691 74,526
U.S. treasury bills    
Fair Value    
Investments at fair value 3,000 4,300
Government-sponsored enterprises securities    
Fair Value    
Investments at fair value 3,655 16,459
Corporate bonds and commercial paper    
Fair Value    
Investments at fair value 55,937 44,271
Fair value inputs Level 1    
Fair Value    
Investments at fair value 5,099 9,496
Fair value inputs Level 2    
Fair Value    
Investments at fair value 62,592 65,030
Fair value measurements recurring | Money market funds | Estimate Of Fair Value Fair Value Disclosure    
Fair Value    
Investments at fair value 5,099 9,496
Fair value measurements recurring | U.S. treasury bills | Estimate Of Fair Value Fair Value Disclosure    
Fair Value    
Investments at fair value 3,000 4,300
Fair value measurements recurring | Government-sponsored enterprises securities | Estimate Of Fair Value Fair Value Disclosure    
Fair Value    
Investments at fair value 3,655 16,459
Fair value measurements recurring | Corporate bonds and commercial paper | Estimate Of Fair Value Fair Value Disclosure    
Fair Value    
Investments at fair value 55,937 44,271
Fair value measurements recurring | Fair value inputs Level 1 | Money market funds    
Fair Value    
Investments at fair value 5,099 9,496
Fair value measurements recurring | Fair value inputs Level 2 | U.S. treasury bills    
Fair Value    
Investments at fair value 3,000 4,300
Fair value measurements recurring | Fair value inputs Level 2 | Government-sponsored enterprises securities    
Fair Value    
Investments at fair value 3,655 16,459
Fair value measurements recurring | Fair value inputs Level 2 | Corporate bonds and commercial paper    
Fair Value    
Investments at fair value $ 55,937 $ 44,271