Annual report pursuant to Section 13 and 15(d)

FAIR VALUE (Details)

v3.8.0.1
FAIR VALUE (Details) - Fair value measurements recurring - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Estimate Of Fair Value Fair Value Disclosure    
Fair Value    
Investments at fair value $ 115,169 $ 74,526
Money market funds | Estimate Of Fair Value Fair Value Disclosure    
Fair Value    
Investments at fair value 2,795 9,496
U.S. treasury bills | Estimate Of Fair Value Fair Value Disclosure    
Fair Value    
Investments at fair value 6,726 4,300
Government-sponsored enterprises securities | Estimate Of Fair Value Fair Value Disclosure    
Fair Value    
Investments at fair value 7,826 16,459
Corporate bonds and commercial paper | Estimate Of Fair Value Fair Value Disclosure    
Fair Value    
Investments at fair value 97,822 44,271
Fair value inputs Level 1    
Fair Value    
Investments at fair value 2,795 9,496
Fair value inputs Level 1 | Money market funds    
Fair Value    
Investments at fair value 2,795 9,496
Fair value inputs Level 2    
Fair Value    
Investments at fair value 112,374 65,030
Fair value inputs Level 2 | U.S. treasury bills    
Fair Value    
Investments at fair value 6,726 4,300
Fair value inputs Level 2 | Government-sponsored enterprises securities    
Fair Value    
Investments at fair value 7,826 16,459
Fair value inputs Level 2 | Corporate bonds and commercial paper    
Fair Value    
Investments at fair value $ 97,822 $ 44,271